The name follows IUPAC rules: Cation is named first → Potassium. The complex anion [CoF₆]³⁻ is named next. Fluoride is the ligand (fluoro), and cobalt’s oxidation state is determined: K₃ means three K⁺ ions = +3 total charge. The complex anion is –3 total, so cobalt must be in +3 oxidation state. Name: Potassium hexafluorocobaltate(III) .

The name follows IUPAC rules: Cation is named first → Potassium. The complex anion [CoF₆]³⁻ is named next. Fluoride is the ligand (fluoro), and cobalt’s oxidation state is determined: K₃ means three K⁺ ions = +3 total charge. The complex anion is –3 total, so cobalt must be in +3 oxidation state. Name: Potassium hexafluorocobaltate(III) .

Think of D and F block chemistry as Level 3 in a video game — you need to unlock Levels 1 and 2 first. If your basics aren’t solid, here’s what you should cover before tackling them: 1. Periodic Table Basics Understand how elements are arranged and what defines blocks (s, p, d, f). This helps you grasp why transition metals sit in the middle, why their properties are “in-between” metals and nonmetals, and why they’re called transition elements . 2. Atomic Structure & Electron Configuration D-block chemistry revolves around the filling and emptying of (n–1)d and ns orbitals. You need to know: How to write electron configurations. The role of Hund’s rule and Aufbau principle. Why Cr and Cu have unusual configurations. 3. Periodic Trends Ionic radius, ionization energy, electronegativity trends — these control oxidation states and complex stability. 4. Chemical Bonding Basics of covalent, ionic, and coordinate covalent bonding — especially how lone pairs work. 5. Oxidation Numbers Since transition metals have multiple oxidation states, knowing how to assign oxidation numbers is a must.Once you’ve mastered these, the D and F block chapters will feel logical rather than overwhelming. Exam tip: If you’re weak in basics, dedicate 1–2 days to just brushing up on Periodic Table trends and electron configurations — these two alone cover half the conceptual weight in D and F block questions.

Think of D and F block chemistry as Level 3 in a video game — you need to unlock Levels 1 and 2 first. If your basics aren’t solid, here’s what you should cover before tackling them: 1. Periodic Table Basics Understand how elements are arranged and what defines blocks (s, p, d, f). This helps you grasp why transition metals sit in the middle, why their properties are “in-between” metals and nonmetals, and why they’re called transition elements . 2. Atomic Structure & Electron Configuration D-block chemistry revolves around the filling and emptying of (n–1)d and ns orbitals. You need to know: How to write electron configurations. The role of Hund’s rule and Aufbau principle. Why Cr and Cu have unusual configurations. 3. Periodic Trends Ionic radius, ionization energy, electronegativity trends — these control oxidation states and complex stability. 4. Chemical Bonding Basics of covalent, ionic, and coordinate covalent bonding — especially how lone pairs work. 5. Oxidation Numbers Since transition metals have multiple oxidation states, knowing how to assign oxidation numbers is a must.Once you’ve mastered these, the D and F block chapters will feel logical rather than overwhelming. Exam tip: If you’re weak in basics, dedicate 1–2 days to just brushing up on Periodic Table trends and electron configurations — these two alone cover half the conceptual weight in D and F block questions.

Technically, yes — but it’s not ideal. Skipping S and P block means missing the comparative framework . Many D-block properties make sense only when you contrast them with S and P blocks: S-block metals have fixed oxidation states; D-block metals have variable oxidation states. P-block elements form covalent compounds; D-block elements tend to form colored complexes. If you skip them, you’ll understand facts but not patterns. That’s like reading the middle of a story without knowing the beginning. Recommendation: If you’re short on time before exams, you can start D-block, but skim through the S and P block general properties chapters. At the very least, know the trends in metallic character, atomic size, and electronegativity from S to P to D block.

Technically, yes — but it’s not ideal. Skipping S and P block means missing the comparative framework . Many D-block properties make sense only when you contrast them with S and P blocks: S-block metals have fixed oxidation states; D-block metals have variable oxidation states. P-block elements form covalent compounds; D-block elements tend to form colored complexes. If you skip them, you’ll understand facts but not patterns. That’s like reading the middle of a story without knowing the beginning. Recommendation: If you’re short on time before exams, you can start D-block, but skim through the S and P block general properties chapters. At the very least, know the trends in metallic character, atomic size, and electronegativity from S to P to D block.

Before diving into Coordination Compounds, be comfortable with: D-block general properties (oxidation states, ionic sizes, magnetic behavior). Chemical bonding basics (especially coordinate bonds). Nomenclature rules — since naming coordination compounds follows strict IUPAC patterns. Hybridization and VSEPR theory — needed to predict geometries of complexes.

Before diving into Coordination Compounds, be comfortable with: D-block general properties (oxidation states, ionic sizes, magnetic behavior). Chemical bonding basics (especially coordinate bonds). Nomenclature rules — since naming coordination compounds follows strict IUPAC patterns. Hybridization and VSEPR theory — needed to predict geometries of complexes.

NEET is a speed-and-accuracy game, and the D & F block elements chapter is a low-weightage but high-return topic. While it may only contribute 3–4 questions, these are often direct and fact-based — meaning you can secure full marks if your revision method is strategic. Step 1: Understand, Don’t Just Memorize Trends Start with the general properties : Electronic configuration of each series (3d, 4d, 5d for D-block; 4f, 5f for F-block). Trends in atomic/ionic radii, ionization enthalpy, oxidation states, and magnetic properties. Once you understand why properties change across a period or down a group, memorizing becomes 10x easier. For example: Atomic radius decreases slightly across the D-block because of increased nuclear charge and ineffective shielding by d-electrons. Step 2: Focus on High-Yield Topics for NEET Color and magnetic properties of complexes (Ligand Field Theory basics). Biological importance of metals (e.g., hemoglobin, chlorophyll). Important compounds like KMnO₄, K₂Cr₂O₇ — preparation, properties, and uses. Lanthanoid contraction and its consequences. Step 3: Use Active Recall Instead of passively reading, try question-answer cards . Example: Q: Why is Sc³⁺ colorless? A: No unpaired d-electrons → no d–d transitions → colorless. Step 4: Integrate Coordination Chemistry Concepts Since many NEET questions blend D-block with Coordination Compounds, study them together. Example: “Which ion in the D-block forms a square planar complex with CN⁻ ligands?” — This needs both D-block knowledge and hybridization concepts. Step 5: Mock Tests & NCERT Mastery 90% of NEET inorganic chemistry comes directly from NCERT lines — so underline every fact in the NCERT chapter. Then, attempt 30–40 MCQs daily from past NEET papers and similar mock tests. NEET Exam Hack for D & F Block: The quickest marks in this topic come from: Oxidation state patterns. Lanthanoid contraction facts. Color/magnetism of complexes. Important reactions of KMnO₄ & K₂Cr₂O₇.

NEET is a speed-and-accuracy game, and the D & F block elements chapter is a low-weightage but high-return topic. While it may only contribute 3–4 questions, these are often direct and fact-based — meaning you can secure full marks if your revision method is strategic. Step 1: Understand, Don’t Just Memorize Trends Start with the general properties : Electronic configuration of each series (3d, 4d, 5d for D-block; 4f, 5f for F-block). Trends in atomic/ionic radii, ionization enthalpy, oxidation states, and magnetic properties. Once you understand why properties change across a period or down a group, memorizing becomes 10x easier. For example: Atomic radius decreases slightly across the D-block because of increased nuclear charge and ineffective shielding by d-electrons. Step 2: Focus on High-Yield Topics for NEET Color and magnetic properties of complexes (Ligand Field Theory basics). Biological importance of metals (e.g., hemoglobin, chlorophyll). Important compounds like KMnO₄, K₂Cr₂O₇ — preparation, properties, and uses. Lanthanoid contraction and its consequences. Step 3: Use Active Recall Instead of passively reading, try question-answer cards . Example: Q: Why is Sc³⁺ colorless? A: No unpaired d-electrons → no d–d transitions → colorless. Step 4: Integrate Coordination Chemistry Concepts Since many NEET questions blend D-block with Coordination Compounds, study them together. Example: “Which ion in the D-block forms a square planar complex with CN⁻ ligands?” — This needs both D-block knowledge and hybridization concepts. Step 5: Mock Tests & NCERT Mastery 90% of NEET inorganic chemistry comes directly from NCERT lines — so underline every fact in the NCERT chapter. Then, attempt 30–40 MCQs daily from past NEET papers and similar mock tests. NEET Exam Hack for D & F Block: The quickest marks in this topic come from: Oxidation state patterns. Lanthanoid contraction facts. Color/magnetism of complexes. Important reactions of KMnO₄ & K₂Cr₂O₇.

Board exams are not just about knowing they’re about writing answers exactly the way the examiner expects. In D & F block, concept clarity + crisp presentation is the key. 1. Prioritize NCERT Line-by-Line Reading Boards rarely go beyond NCERT for inorganic chemistry. Mark every trend, table, and reaction mechanism. Pay special attention to examples and exceptions (like Cu⁺ being unstable in aqueous solution). 2. Use Structured Notes For each series (3d, 4d, 5d, lanthanoids, actinoids), prepare a one-page summary including: Electronic configuration. Oxidation states and stability trends. Magnetic properties. Color and ligand preferences. 3. Practice Reactions with Balanced Equations In exams, incomplete equations lose marks. For example: 2MnO₄⁻ + 10I⁻ + 16H⁺→ 2Mn²⁺ + 5I₂ + 8H₂O. Write them repeatedly to lock them in memory. 4. Visual Memory Tools Draw periodic table segments highlighting only D & F block elements. Use color codes for oxidation states and magnetic properties. 5. Previous Year Questions Boards often repeat questions, sometimes verbatim. Practice at least 10 years of previous papers for D & F block.

Board exams are not just about knowing they’re about writing answers exactly the way the examiner expects. In D & F block, concept clarity + crisp presentation is the key. 1. Prioritize NCERT Line-by-Line Reading Boards rarely go beyond NCERT for inorganic chemistry. Mark every trend, table, and reaction mechanism. Pay special attention to examples and exceptions (like Cu⁺ being unstable in aqueous solution). 2. Use Structured Notes For each series (3d, 4d, 5d, lanthanoids, actinoids), prepare a one-page summary including: Electronic configuration. Oxidation states and stability trends. Magnetic properties. Color and ligand preferences. 3. Practice Reactions with Balanced Equations In exams, incomplete equations lose marks. For example: 2MnO₄⁻ + 10I⁻ + 16H⁺→ 2Mn²⁺ + 5I₂ + 8H₂O. Write them repeatedly to lock them in memory. 4. Visual Memory Tools Draw periodic table segments highlighting only D & F block elements. Use color codes for oxidation states and magnetic properties. 5. Previous Year Questions Boards often repeat questions, sometimes verbatim. Practice at least 10 years of previous papers for D & F block.

For NEET/JEE/Boards, focus on: Definition of ligand, coordination number, coordination entity. IUPAC nomenclature rules. Types of ligands (monodentate, bidentate, polydentate, ambidentate). Isomerism (structural and stereoisomerism). Bonding theories (VBT and CFT). Applications (medicine, industry, biology).

For NEET/JEE/Boards, focus on: Definition of ligand, coordination number, coordination entity. IUPAC nomenclature rules. Types of ligands (monodentate, bidentate, polydentate, ambidentate). Isomerism (structural and stereoisomerism). Bonding theories (VBT and CFT). Applications (medicine, industry, biology).

In Class 12 Chemistry, the d-block elements are taught as part of the transition metals section of the periodic table. They occupy the central part of the periodic table — Groups 3 to 12 — and are characterized by having their d-orbitals partially filled in at least one common oxidation state. Classification by Periods and Series The d-block elements are divided into four series based on the principal quantum number of the (n–1)d orbitals being filled: 3d series (First Transition Series) – Period 4, from Scandium (Sc) to Zinc (Zn). 4d series (Second Transition Series) – Period 5, from Yttrium (Y) to Cadmium (Cd). 5d series (Third Transition Series) – Period 6, from Hafnium (Hf) to Mercury (Hg). 6d series (Fourth Transition Series) – Period 7, starting from Rutherfordium (Rf). (Only some are well-studied due to radioactivity.) Groups and Special Members Group 3 elements : Sc, Y, La/Ac (sometimes debated due to periodic table arrangements). Group 11 : Cu, Ag, Au — known as coinage metals. Group 12 : Zn, Cd, Hg — often considered “pseudo-transition” metals because their d-orbitals are completely filled in the most stable oxidation state. Why Classification Matters for Exams This classification helps in spotting trends — like magnetic properties, color, oxidation states — because these trends repeat within a series. For example, all members of Group 6 (Cr, Mo, W) tend to have high oxidation states (+6).

In Class 12 Chemistry, the d-block elements are taught as part of the transition metals section of the periodic table. They occupy the central part of the periodic table — Groups 3 to 12 — and are characterized by having their d-orbitals partially filled in at least one common oxidation state. Classification by Periods and Series The d-block elements are divided into four series based on the principal quantum number of the (n–1)d orbitals being filled: 3d series (First Transition Series) – Period 4, from Scandium (Sc) to Zinc (Zn). 4d series (Second Transition Series) – Period 5, from Yttrium (Y) to Cadmium (Cd). 5d series (Third Transition Series) – Period 6, from Hafnium (Hf) to Mercury (Hg). 6d series (Fourth Transition Series) – Period 7, starting from Rutherfordium (Rf). (Only some are well-studied due to radioactivity.) Groups and Special Members Group 3 elements : Sc, Y, La/Ac (sometimes debated due to periodic table arrangements). Group 11 : Cu, Ag, Au — known as coinage metals. Group 12 : Zn, Cd, Hg — often considered “pseudo-transition” metals because their d-orbitals are completely filled in the most stable oxidation state. Why Classification Matters for Exams This classification helps in spotting trends — like magnetic properties, color, oxidation states — because these trends repeat within a series. For example, all members of Group 6 (Cr, Mo, W) tend to have high oxidation states (+6).

The term “transition elements” comes from the fact that these elements represent a transition in properties between the electropositive s-block metals and the electronegative p-block nonmetals in the periodic table. Key reasons for the name: Electronic Transition: The filling of d-orbitals marks a gradual change in electron configuration between blocks. Property Transition: Metallic character, oxidation states, and bonding types change gradually across the series. Chemical Versatility: They can show both metallic properties (conductivity, malleability) and nonmetal-like behavior (forming covalent bonds). An example: Titanium sits between calcium (highly reactive s-block metal) and gallium (a p-block element with more nonmetallic tendencies) — and shows mixed characteristics. Textbook Definition: A transition element is defined as “an element that has a partially filled d-orbital in its ground state or in any one of its common oxidation states.”

The term “transition elements” comes from the fact that these elements represent a transition in properties between the electropositive s-block metals and the electronegative p-block nonmetals in the periodic table. Key reasons for the name: Electronic Transition: The filling of d-orbitals marks a gradual change in electron configuration between blocks. Property Transition: Metallic character, oxidation states, and bonding types change gradually across the series. Chemical Versatility: They can show both metallic properties (conductivity, malleability) and nonmetal-like behavior (forming covalent bonds). An example: Titanium sits between calcium (highly reactive s-block metal) and gallium (a p-block element with more nonmetallic tendencies) — and shows mixed characteristics. Textbook Definition: A transition element is defined as “an element that has a partially filled d-orbital in its ground state or in any one of its common oxidation states.”

The name “d-block” simply means the block of the periodic table where the last electron enters a d-orbital . The “d” refers to the type of atomic orbital (d-orbitals have l = 2 in quantum mechanics). Quick facts: Position: Groups 3–12, between s-block and p-block. Outer electron configuration: (n–1)d¹–¹⁰ ns¹–². Signature properties: Variable oxidation states, colored compounds, catalytic behavior, tendency to form complexes. In Real Life: The properties of d-block metals explain why they dominate industries: Iron in construction. Platinum in catalytic converters. Copper in electrical wiring. Titanium in aerospace.

The name “d-block” simply means the block of the periodic table where the last electron enters a d-orbital . The “d” refers to the type of atomic orbital (d-orbitals have l = 2 in quantum mechanics). Quick facts: Position: Groups 3–12, between s-block and p-block. Outer electron configuration: (n–1)d¹–¹⁰ ns¹–². Signature properties: Variable oxidation states, colored compounds, catalytic behavior, tendency to form complexes. In Real Life: The properties of d-block metals explain why they dominate industries: Iron in construction. Platinum in catalytic converters. Copper in electrical wiring. Titanium in aerospace.

D-Block and Coordination Compounds Class 12 Notes – Transition Elements & Complexes

Q: If you think of your Class 12 Chemistry syllabus as a series of puzzle pieces , D-block elements and Coordination compounds are two pieces that fit very closely together — but one of them makes the other much easier to understand.Here’s why: D-block elements give you the foundation : their position in the periodic table, electronic configurations, variable oxidation states, magnetic properties, and color — all of which directly explain why and how they form coordination compounds . If you skip straight to Coordination Compounds, you’ll still learn about ligands, coordination numbers, and IUPAC naming, but you’ll be missing the “why these compounds exist in the first place” part.For example, the fact that Fe³⁺ forms a stable [Fe(CN)₆]³⁻ complex becomes far more intuitive once you already understand that Fe³⁺ has a small ionic radius, high charge density, and partially filled d-orbitals. Without this base, you’ll just be memorizing facts instead of understanding concepts. Best practice for students (NEET/JEE/Boards) Step 1: Study D-block elements first — focus on general characteristics, trends across the series, and exceptions. Step 2: Move to Coordination Compounds — here you’ll see those properties in action. Step 3: Link back and forth — as you learn ligand field theory, recall what you learned about oxidation states, and you’ll find it easier to answer application-based questions. Exam insight: Coordination Compounds often carry 5–8 marks in Board Exams and multiple direct MCQs in NEET/JEE. Understanding the D-block first can boost your score because you’ll handle conceptual questions faster.

If you think of your Class 12 Chemistry syllabus as a series of puzzle pieces , D-block elements and Coordination compounds are two pieces that fit very closely together — but one of them makes the other much easier to understand.Here’s why: D-block elements give you the foundation : their position in the periodic table, electronic configurations, variable oxidation states, magnetic properties, and color — all of which directly explain why and how they form coordination compounds . If you skip straight to Coordination Compounds, you’ll still learn about ligands, coordination numbers, and IUPAC naming, but you’ll be missing the “why these compounds exist in the first place” part.For example, the fact that Fe³⁺ forms a stable [Fe(CN)₆]³⁻ complex becomes far more intuitive once you already understand that Fe³⁺ has a small ionic radius, high charge density, and partially filled d-orbitals. Without this base, you’ll just be memorizing facts instead of understanding concepts. Best practice for students (NEET/JEE/Boards) Step 1: Study D-block elements first — focus on general characteristics, trends across the series, and exceptions. Step 2: Move to Coordination Compounds — here you’ll see those properties in action. Step 3: Link back and forth — as you learn ligand field theory, recall what you learned about oxidation states, and you’ll find it easier to answer application-based questions. Exam insight: Coordination Compounds often carry 5–8 marks in Board Exams and multiple direct MCQs in NEET/JEE. Understanding the D-block first can boost your score because you’ll handle conceptual questions faster.

Transition Elements & Coordination Compounds – Study Notes, Properties & Examples

Master the concepts of Transition Elements and Coordination Compounds with our comprehensive guide designed for students preparing for IIT-JEE, NEET, and competitive exams. From the general properties of d-block elements to the advanced theories of bonding, this page covers everything you need in one place.

What are Transition Elements?

Transition elements are d-block elements with a partially filled d-subshell in their atoms or common oxidation states. They are located between the s-block and p-block in the periodic table and exhibit properties intermediate between the two.

Examples: Iron (Fe), Copper (Cu), Nickel (Ni).
Note: Elements like Zn, Cd, and Hg are in the d-block but are not true transition elements due to fully filled d-orbitals (d10 configuration).

General Properties of Transition Metals

Transition metals are known for:

High melting and boiling points due to strong metallic bonding.
Variable oxidation states (e.g., Fe2+ and Fe3+).
Formation of coloured compounds because of d–d electronic transitions.
Magnetic properties like paramagnetism and ferromagnetism.
Ability to form complex ions with ligands via coordinate bonds.
Catalytic activity in industrial reactions (e.g., V2O5 in Contact Process).

Compounds of Transition Metals

Some important examples include:

Potassium Permanganate (KMnO4): Strong oxidizing agent in acidic, neutral, and alkaline mediums.
Potassium Dichromate (K2Cr2O7): Used in volumetric analysis and oxidation reactions.
Copper Sulphate (CuSO4·5H2O): Blue crystalline solid, used in agriculture and electroplating.
Silver Nitrate (AgNO3): Known as lunar caustic, used in photography and analytical chemistry.

Coordination Compounds

Definition: Complexes in which a central metal ion is bonded to molecules/ions (ligands) via coordinate bonds.

Key Terms:

Central Metal Ion – metal atom accepting electron pairs.
Ligands – neutral or anionic species donating lone pairs.
Coordination Number (C.N.) – number of donor atoms directly bonded to the central metal.
Oxidation Number – hypothetical charge on the metal if ligands are removed.

Examples: [Co(NH3)6]3+, [Fe(CN)6]4-, [Cu(NH3)4]2+.

IUPAC Nomenclature of Coordination Compounds

Cations before anions in naming.
Ligands are named alphabetically, with prefixes (di, tri, tetra).
Anionic complexes end in -ate (e.g., ferrate, chromate).
Oxidation state is given in Roman numerals.

Example:
[Cr(H2O)4Cl2]Cl → tetraaquadichlorochromium(III) chloride

Isomerism in Coordination Compounds

Classes include:

Structural Isomerism: Ionization, hydrate, linkage, and coordination isomerism.
Stereoisomerism: Geometrical (cis–trans) and optical isomerism.

Crystal Field Theory (CFT)

Explains splitting of d-orbitals in different ligand geometries:

Octahedral complexes – splitting into t2g (low energy) and eg (high energy) orbitals.
Tetrahedral complexes – reverse splitting order.
Determines high-spin vs. low-spin configurations depending on ligand strength (Spectrochemical Series).

Why Study with Us?

Exam-focused explanations based on FIITJEE and NCERT patterns.
Comprehensive notes for transition elements and coordination compounds.
Detailed solved examples and exercises for practice.
Prepared for IIT-JEE, NEET, and other competitive exams.

INTRODUCTION

1. The elements in which the differentiating electron enters into the penultimate d-orbitals are called d-block elements.

2. The elements with partly filled d-subshell in their atoms or in common oxidation states are known as transition elements.

3. The d-block elements are present in between those of s-block and p-block elements. Because these elements show properties intermediate between the properties of s and p-block elements.

4. The elements of group IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIIIB of periodic table are called transition elements.

5. The general electronic configuration of transition elements are (n-1)d¹^-¹⁰ns⁰^-².

6. The elements with ns²(n-1)d¹⁰ configuration are not transition elements although they belong to d-block, e.g. Zn(3d¹⁰4s²), Cd(4d¹⁰5s²), Hg(5d¹⁰6s²).

7. Cu, Ag and Au are called coinage metals.

8. Zn, Cd and Hg are the last members of each d-block series.

9. First transition series starts from Sc (Z = 21) to Zn (Z = 30) are called 3d-series.

10. Second transition series starts from Y (Z = 39) to Cd (Z = 48) are called
4d-series.

11. Third transition series starts from La (Z=57) and followed by Hf (Z=72) to Hg (Z=80) are called 5d-series.

12. Elements of VIIIB group (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt) are called typical transition elements.

GENERAL PROPERTIES OF TRANSITION ELEMENTS

Metallic Character

All the transition elements are metals. They are less metallic than alkali and alkaline earth metals. In a transition series on moving from left to right, the strength of metallic bond first increases and then decreases.

Conductivity

All the transition metals are good conductors of heat and electricity. Silver is the best conductor of electricity.

Density

Because of small size of their atoms and strong metallic bonding the density and hardness of transition elements are high.

Melting and Boling Points

These elements have high melting and boiling points due to strong interatomic attraction and strong metallic bonding. In a transition series the melting point first increases and then decreases. Melting point of Zn, Cd and Hg are low because of weak metallic bonding in them.

Atomic and Ionic Radii

The atomic and ionic radii of transition elements are smaller than those of s-block elements and larger than those of p-block elements.

Atomic and ionic radii ∝ 1effective nuclear charge

∝ Screening effect

∝ Inter-electronic repulsion

Ionization Energy

The ionization energy (IE) of transition elements are higher than those of s-block elements but lower than p-block elements. In a particular transition series, ionization energy although increases gradually as we move from left to right but this increase is not appreciable.

The increase in ionization energy is due to increase in nuclear charge, the effect of increase in nuclear charge is partly balanced by the increase in screening effect. Consequently, the increase in ionization energy along the period of d-block elements is very small.

Coloured Complexes

The d-orbitals in the transition elements do not have same energy in their complexes.

(i) Under the influence of the ligands attached, the d-orbitals split into two sets of orbitals having slightly different energies.

(ii) In the transition elements which have partially filled d-orbitals, the transition of electrons can take place from one of the lower energy d-orbitals to some higher energy d-orbitals within the same subshell. The energy required for this transition falls in the visible region. So, when white light falls on these complexes they absorb a particular colour from the radiation for the promotion of electron and the complementary colours are emitted. The colour of the complex is due to this emitted radiation.

Magnetic Properties

(i) Diamagnetic substances are feebly repelled by an applied magnetic field. Such substances have no unpaired electrons.

(ii) Paramagnetic substances are attracted by an applied magnetic field. Such substances have at least one unpaired electron.

(iii) Ferromagnetism is a special type of paramagnetism in which permanent magnetic moment is acquired by the substance due to the presence of unpaired electrons.

(iv) The magnetic moment arises from the spin and orbital motions in ions or molecules. It is given as:

u = root n (n + 2) Bohr magneton Where n is number of unpaired electrons.

Formation of Alloy

Since d-block elements have almost similar atomic size, they can mutually substitute one another in the crystal lattice to form alloys, e.g. german silver (Cu, Ni, Zn), nickel steel (2.5 – 5.0% Ni), tungsten steel (6-8% W).

Complex Ion Formation

All transition elements form co-ordination compounds readily. The tendency of the transition metals to form complexes is due to

(i) small size of the metal ions.

(ii) large ionic or nuclear charge.

(iii) available vacant d-orbitals.

(iv) low basicity of the metal ions.

Interstitial Compounds

Transition metals can trap some small atoms like hydrogen, boron, carbon, nitrogen etc. in vacant spaces in their crystal lattice forming interstitial compounds. Carbon and nitrogen always occupy octahedral holes. Hydrogen is smaller and always occupies tetrahedral holes. The composition of these compounds is generally non-stoichiometric, e.g. TiH_1.73, PdH_0.56, VH_0.56 but may approach regular stoichiometry and a regular structure, e.g. TiC and VN. The later transition elements of the first series form non-stoichiometric carbides with irregular structures. Such as Cr₇C₃, which are more reactive than the interstitial carbides of the early transition elements.

COMPOUNDS OF TRANSITION METALS

Potassium Permanganate (KMnO₄)

Preparation

It is prepared by fusing pyrolusite ore either with KOH or K₂CO₃ in presence of atmospheric oxygen.

2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O
2MnO₂ + 2K₂CO₃ + O₂ → 2K₂MnO₄ + 2CO₂

The fused mass is extracted with water and the solution is treated with a current of Cl₂ or O₃ or CO₂.

2K₂MnO₄ + Cl₂ → 2KMnO₄ + 2KCl
2K₂MnO₄ + H₂O + O₃ → 2KMnO₄ + 2KOH + O₂
2K₂MnO₄ + 2CO₂ → 2KMnO₄ + MnO₂ + 2K₂CO₃

Properties

It is a purple coloured crystalline compound, fairly soluble in water.

Effect of heat

2KMnO₄ —Δ→ K₂MnO₄ + MnO₂ + O₂
4KMnO₄ + 4KOH —Δ→ 4K₂MnO₄ + 2H₂O + O₂

Reaction with conc. H₂SO₄

On treatment with conc. H₂SO₄ it forms manganese heptoxide via permanganyl sulphate which decomposes explosively on heating.

2KMnO₄ + 3H₂SO₄ → 2KHSO₄ + (MnO₃)₂SO₄ + 2H₂O
(MnO₃)₂SO₄ + H₂O → Mn₂O₇ + H₂SO₄
Mn₂O₇ → 2MnO₂ + 3⁄2 O₂

Oxidizing behaviour

Potassium permanganate acts as an oxidizing agent in alkaline, neutral or acidic solution.

In alkaline solution: 2KMnO₄ + 2KOH → 2K₂MnO₄ + H₂O + [O]
In neutral solution: 2KMnO₄ + H₂O → 2MnO₂ + 2KOH + 3[O]
In acidic solution: 2KMnO₄ + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 3H₂O + 5[O]

The important oxidation reactions are

(a) In acidic medium:

2MnO₄⁻ + 10Fe²⁺ + 16H⁺ → 10Fe³⁺ + 2Mn²⁺ + 8H₂O
2MnO₄⁻ + 10I⁻ + 16H⁺ → 2Mn²⁺ + 5I₂ + 8H₂O
5H₂C₂O₄ + 2KMnO₄ + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 10CO₂ + 8H₂O
2KMnO₄ + 5SO₂ + 2H₂O → K₂SO₄ + 2MnSO₄ + 2H₂SO₄
2KMnO₄ + 3H₂SO₄ + 5H₂S → K₂SO₄ + 2MnSO₄ + 5S + 8H₂O
2KMnO₄ + 3H₂SO₄ + 10HX → K₂SO₄ + 2MnSO₄ + 8H₂O + 5X₂

(b) In neutral medium:

2KMnO₄ + 3H₂S → 2KOH + 2MnO₂ + 2H₂O + 3S
2KMnO₄ + 3Na₂S₂O₃ + H₂O → 2KOH + 2MnO₂ + 3Na₂SO₄ + 3S

(c) In alkaline medium:

2KMnO₄ + KI + H₂O → 2KOH + 2MnO₂ + KIO₃
Alkene + H₂O + [O] (alk. KMnO₄) → vicinal diol (–CH₂OH–CH₂OH)

Potassium Dichromate (K₂Cr₂O₇)
(i) It is an orange-red coloured crystalline compound.
(ii) It is moderately soluble in cold water but freely soluble in hot water. It melts at 398°C.

Effect of heat
(i)

2K₂Cr₂O₇ —Δ→ 2K₂CrO₄ + Cr₂O₃ + 3/2 O₂
 (orange) (yellow)

(ii)

K₂Cr₂O₇ + 2KOH —Δ→ 2K₂CrO₄ + H₂O
 (orange) (yellow)

On acidifying yellow colour again changes to orange.

2CrO₄²⁻ + 2H⁺ → 2Cr₂O₇²⁻ + H₂O
(yellow) (orange)

Chromyl chloride test (test of chloride ion)
When a mixture of a metal chloride and potassium dichromate is heated with conc. H₂SO₄, orange-red vapours of chromyl chloride are evolved.

K₂Cr₂O₇ + 6H₂SO₄ + 4NaCl → 2KHSO₄ + 4NaHSO₄ + 2CrO₂Cl₂ + 3H₂O
 (chromyl chloride)

When chromyl chloride vapours are passed through NaOH solution, yellow coloured solution is obtained.

CrO₂Cl₂ + 4NaOH → Na₂CrO₄ + 2NaCl + 2H₂O
 (yellow solution)

When lead acetate and acetic acid are added into this yellow solution, a yellow precipitate is obtained.

Na₂CrO₄ + (CH₃COO)₂Pb —CH₃COOH→ PbCrO₄ + 2CH₃COONa
 (yellow ppt.)

Ferrous Sulphate (FeSO₄·7H₂O) or Green Vitriol

It is also known as harakasis.

Preparation

Commercially, it is prepared by the slow oxidation of iron pyrites in the presence of air and moisture.

2FeS₂ + 2H₂O + 7O₂ → 2FeSO₄ + 2H₂SO₄

On crystallization, green crystals are formed.

Properties

Due to atmospheric oxidation the crystals acquire brownish yellow colour due to formation of basic ferric sulphate.

4FeSO₄ + 2H₂O + O₂ → 4Fe(OH)·SO₄

basic ferric sulphate

(i) Action of heat

FeSO₄·7H₂O — 300°C, −7H₂O → 2FeSO₄ → high temperature → Fe₂O₃ + SO₂ + SO₃

(green) → (white)

(ii) It reduces gold chloride to gold

AuCl₃ + 3FeSO₄ ⟶ Au + Fe₂(SO₄)₃ + FeCl₃

(iii) It combines with KCN (excess) forming potassium ferrocyanide.

FeSO₄ + 2KCN ⟶ Fe(CN)₂ + K₂SO₄

Fe(CN)₂ + 4KCN ⟶ K₄Fe(CN)₆

FeSO₄ + 6KCN ⟶ K₄[Fe(CN)₆] + K₂SO₄

(iv) A cold solution of FeSO₄ absorbs nitric oxide forming dark brown addition compound.

FeSO₄ + NO ⟶ FeSO₄.NO
nitroso ferrous sulphate (brown)

Silver Nitrate (AgNO₃) or Lunar Caustic

Preparation

3Ag + 4HNO₃ (dil.) —Δ—> 3AgNO₃ + NO + 2H₂O

Properties

It is a colourless crystalline compound soluble in water and alcohol.

On heating above its melting point, it decomposes.

2AgNO₃ ⟶ 2AgNO₂ + O₂

When heated to red heat, it further decomposes to metallic silver.

2AgNO₃ ⟶ 2Ag + 2NO₂ + O₂

Reaction	Equation
Halides	AgNO₃ + NaX ⟶ AgX↓ + NaNO₃
Phosphate	3AgNO₃ + Na₃PO₄ ⟶ Ag₃PO₄↓ + 3NaNO₃
Sulphide	2AgNO₃ + Na₂S ⟶ Ag₂S↓ + 2NaNO₃ (black ppt.)
Chromates	2AgNO₃ + K₂CrO₄ ⟶ Ag₂CrO₄↓ + 2KNO₃ (brick red ppt.)
Thiocyanates	AgNO₃ + NaCNS ⟶ AgCNS↓ + NaNO₃
Thiosulphate	2AgNO₃ + Na₂S₂O₃ ⟶ Ag₂S₂O₃↓ + 2NaNO₃ (white ppt.) Ag₂S₂O₃ + H₂O ⟶ Ag₂S↓ + H₂SO₄ (black)
Tollen's reagent	2AgNO₃ + 2NH₄OH ⟶ Ag₂O↓ + 2NH₄NO₃ + H₂O Ag₂O + 2NH₄NO₃ + 2NH₄OH ⟶ 2[Ag(NH₃)₂]NO₃ + 3H₂O

Reactions Involving Silver Compounds (Photographic Film)

Reactions Involving Silver Compounds (Photographic Film)

It converts glucose to gluconic acid.

Ag₂O + C6H12O6 → 2Ag + C6H12O7

It oxidizes HCHO to HCOOH.

Ag₂O + HCHO → 2Ag + HCOOH

The reaction used in photographic film

The treatment of the exposed photographic film with a reducing agent is called developing of the film, and the chemicals used for developing as reducing agents are called developers (e.g., potassium ferrous oxalate, or an alkaline solution of pyrogallol or an alkaline solution of quinol).

Quinol + 2AgBr → Quinone + 2Ag + 2HBr

In order to make the image permanent, it is necessary to remove the unreduced AgBr from the surface of the developed film, known as fixing of the image. This is done by dipping the developed film in hypo solution (sodium thiosulphate).

AgBr + 2Na2S2O3 → Na3[Ag(S2O3)2] + NaBr

(sodium argento thiosulphate)

Cuprous Chloride (Cu₂Cl₂)

Preparation

By passing SO₂ through the solution containing copper sulphate and sodium chloride.

2CuSO₄ + 2NaCl + 2H₂O + SO₂ → Cu₂Cl₂ + Na₂SO₄ + 2H₂SO₄
By boiling copper sulphate solution with excess of copper turnings in the presence of HCl.
CuSO₄ + 2HCl → CuCl₂ + H₂SO₄
CuCl₂ + Cu → Cu₂Cl₂

Properties

It is a white solid, insoluble in water.

Cu₂Cl₂ + 4HCl → 2H₂CuCl₃
Cu₂Cl₂ + 6HCl → 2H₃CuCl₄
Cu₂Cl₂ + 2H₂O + O₂ → 2[CuCl₂·Cu(OH)₂] (basic cupric chloride)
Cu₂Cl₂ + 2NaOH → Cu₂O (yellow, changing to red) + 2NaCl + 2H₂O
Cu₂Cl₂ + H₂S → Cu₂S + 2HCl

Copper Sulphate (CuSO₄·5H₂O) or Blue VitriolPreparation

CuO + H₂SO₄ → CuSO₄ + H₂O
Cu(OH)₂ + H₂SO₄ → CuSO₄ + 2H₂O
Cu(OH)₂, CuCO₃ + 2H₂SO₄ → 2CuSO₄ + 3H₂O + CO₂

Properties

It is a blue crystalline compound and is partially soluble in water.

Effect of heating
- CuSO₄·5H₂O (blue) → (100°C) CuSO₄·3H₂O (pale blue)
- CuSO₄·3H₂O → (200°C) CuSO₄·H₂O (bluish white)
- CuSO₄·H₂O → (220°C) CuSO₄ (white)
- CuSO₄ —(720°C)—> CuO + SO₃
- SO₃ → SO₂ + ½ O₂
Action of NH₄OH
- CuSO₄ + 2NH₄OH → Cu(OH)₂↓ + (NH₄)₂SO₄
- Cu(OH)₂ + 2NH₄OH + (NH₄)₂SO₄ → [Cu(NH₃)₄]SO₄ + 4H₂O
- The complex formed is known as Schweizer’s reagent. It is used in the cuprammonium process for the manufacturing of rayon.
Action of potassium ferrocyanide
- Reddish brown precipitate of cupric ferrocyanide is formed (test of Cu²⁺ ion).
- 2CuSO₄ + K₄[Fe(CN)₆] → Cu₂[Fe(CN)₆] + 2K₂SO₄
Action of potassium thiocyanate
- CuSO₄ + 2KCNS → Cu(CNS)₂ + K₂SO₄
- 2CuSO₄ + 2KCNS + SO₂ + 2H₂O → Cu₂(CNS)₂ + K₂SO₄ + H₂SO₄
- (cupric thiocyanate)
Action of potassium iodide
- [CuSO₄ + 2KI → CuI₂ + K₂SO₄] × 2
- 2CuI₂ → Cu₂I₂ + I₂
- 2CuSO₄ + 4KI → Cu₂I₂ + 2K₂SO₄ + I₂

Zinc Sulphate (ZnSO₄.7H₂O) or White Vitriol Preparation

Zn + H₂SO₄ → ZnSO₄ + H₂
ZnO + H₂SO₄ → ZnSO₄ + H₂O
ZnCO₃ + H₂SO₄ → ZnSO₄ + H₂O + CO₂

The solution on concentration and crystallization below 39°C gives colourless crystals of zinc sulphate (ZnSO₄.7H₂O)

Properties

It is a colourless, crystalline solid. It is an efflorescent substance and easily soluble in water.

Action of Heat:
ZnSO₄·7H₂O →_39-70°C ZnSO₄·6H₂O →_70°C ZnSO₄·H₂O →_280°C ZnSO₄ (anhydrous) →_800°C ZnO + SO₃ + O₂
ZnSO₄ + 2NaOH → Zn(OH)₂↓ + Na₂SO₄
White precipitate of Zn(OH)₂ dissolves in excess of NaOH.
Zn(OH)₂ + 2NaOH → Na₂ZnO₂ + 2H₂O (sodium zincate)
4ZnSO₄ + 4Na₂CO₃ + 3H₂O → ZnCO₃·3Zn(OH)₂ (basic zinc carbonate) + 4Na₂SO₄ + 3CO₂
ZnSO₄ + 2NaHCO₃ → ZnCO₃ + Na₂SO₄ + H₂O + CO₂
With alkali metal sulphates and (NH₄)₂SO₄, it forms double sulphates such as K₂SO₄·ZnSO₄·6H₂O
It is used for the preparation of lithopone (ZnS + BaSO₄), which is also known as Charlton white and used as a pigment for white paints.

Mercurous chloride (Hg₂Cl₂) or Calomel
Preparation

Hg₂(NO₃)₂ + 2HCl → Hg₂Cl₂ + 2HNO₃
Hg₂(NO₃)₂ + 2NaCl → Hg₂Cl₂ + 2NaNO₃
Hg + HgCl₂ → Hg₂Cl₂
2HgCl₂ + 2H₂O + SO₂ → Hg₂Cl₂ + H₂SO₄ + 2HCl

Properties

It is an amorphous, odourless white powder. It is sparingly soluble in water. It sublimes at 373°C.

It dissolves in chlorine water, aqua regia or a mixture of KClO₃ and concentrated HCl.
Hg₂Cl₂ + 2Cl₂ → 2HgCl₂
Hg₂Cl₂ →_Δ Hg + HgCl₂
It turns black when treated with NH₄OH.
Hg₂Cl₂ + 2NH₄OH → Hg + Hg + NH₄Cl + 2H₂O (black)
It is reduced to metallic mercury with SnCl₂.
SnCl₂ + Hg₂Cl₂ → 2Hg + SnCl₄
It absorbs ammonia gas and forms an addition compound.
Hg₂Cl₂ + 2NH₃ → Hg₂Cl₂·2NH₃

Mercuric Chloride (HgCl₂) or Corrosive Sublimate

Preparation
(i) Hg + Cl₂ —Δ→ HgCl₂
(ii) HgO + 2HCl → HgCl₂ + H₂O
(iii) HgSO₄ + 2NaCl → HgCl₂ + Na₂SO₄

Properties
It is a white crystalline compound. It has corrosive action, highly poisonous. It sublimes on heating.

(i) On addition of KI it gives scarlet precipitate of mercuric iodide, which is dissolved in excess of KI.
HgCl₂ + 2KI → 2KCl + HgI₂ ↓ (scarlet red)
HgI₂ + 2KI → K₂HgI₄ (Nessler’s reagent)

[Note: Alkaline solution of K₂HgI₄ is known as Nessler’s reagent, used to detect the ammonium cation.]

(ii) When H₂S is passed through its solution it forms a black precipitate of mercuric sulphide.
HgCl₂ + H₂S → HgS ↓ + 2HCl (black)

(iii) It is reduced by stannous chloride.
2HgCl₂ + SnCl₂ → Hg₂Cl₂ (white) + SnCl₄
Hg₂Cl₂ + SnCl₂ → 2Hg (black) + SnCl₄

(iv) HgCl₂ when heated with Na₂CO₃ or NaOH in solution gives a yellow precipitate of mercuric oxide.
HgCl₂ + Na₂CO₃ → HgO + 2NaCl + CO₂
HgCl₂ + 2NaOH → HgO + 2NaCl + H₂O

CO-ORDINATION COMPOUNDS

Co-ordination compounds are the compounds in which the central metal atom is linked to ions or neutral molecules by co-ordinate bonds, e.g. [Cr(H₂O)₅Cl]²⁺. If the species thus formed as given above carries positive charge, it is called a complex ion.

TERMINOLOGY

The important terms used in the study of coordination compounds are mentioned below.

Central Metal Ion

It is an acceptor atom containing vacant orbitals to which a fixed number of ligands are attached via co-ordinate bonds in definite geometrical arrangement.

Ligands

It is an ion or molecule capable of donating a pair of electrons to the central atom via a donor

atom.

Unidentate ligands. Ligands with only one donor atom, e.g. NH₃, Cl^-, F^- etc.

Bidentate ligands. Ligands with two donor atoms, e.g. ethylenediamine, C₂O₄²^- (oxalate ion) etc.

Tridentate ligands. Ligands which have three donor atoms per ligand, e.g. (dien) diethyl triamine.

Hexadentate ligands. Ligands which have six donor atoms per ligand, e.g. EDTA.

Co-ordination Sphere

Complex ion enclosed in square bracket, it behaves as a single unit.

Ionization Sphere

Part of compound present outside co-ordination sphere, e.g

ionization sphere

Example: The number of ions given by [Co (NH₃)₄]Cl₃ in aqueous solution is

(A) 2

(B) 3

(D) 4

Solution: (D).

Co-ordination sphere does not give its ions individually of its metal atom ions.

Co-ordination Number

It is the total number of ligands attached to the central metal atom through co-ordinate bonds or the number of atoms of a ligand attached to the same central atom, e.g. hexadentate ligand should be counted as forming six co-ordinate bonds.

Oxidation number

It is the charge which the central atom appears to have if all the ligands are removed along with the electron pairs that are shared with the central atom.

[Cr(H₂O)₄Cl₂]⁺

x + (4 × 0) + (−1 × 2) = +1

x + 0 – 2 = +1 = 3

[ ∵ the ligand H₂O is neutral and 2 Cl⁻ carries −2 charge ]

Chelating Ligands
Multidentate ligand simultaneously co-ordinating to a metal ion through more than one site is called chelating ligand. These ligands produce a ring like structure called chelate. Chelation increases the stability of complex. This effect is called chelation effect.

EAN (Effective Atomic Number)
It is the resultant number of electrons with the metal atom or ion after gaining electrons from the donor atoms of the ligands. EAN generally coincides with atomic number of next inert gas in some cases.
EAN = (Atomic number of metal) – (Number of e⁻ lost in ion formation) + (Number of e⁻ gained from donors)
For example, in K₄[Fe(CN)₆]

EAN = (26 – 2) + (6 × 2) = 24 + 12 = 36, i.e. atomic number of Kr.

RULES OF NOMENCLATURE

Cations are written first followed by anions. The name starts with a small letter and is written as one word.
Negative ligands end in –o, positive ligands end in –ium while neutral ligands have no special ending. Organic ligands are written as such in brackets.
For complicated ligands we use abbreviation, e.g. NO₂⁺ (nitronium), NO⁺ (nitrosonium), CH₃ (methyl), en (ethylenediamine), CH₃COO⁻ (acetate) etc.
For more than one ligand of the same type the number is indicated by di, tri, tetra. For ligands having words like ethylenediamine, the number is indicated by bis, tris, tetrakis, etc. For example, four CN are written as tetracyano while four(en) are written as tetrakis (ethylenediamine).
For cationic complex or non ionic complex, name of metal is written as such,
e.g. [Ni(NH₃)₄]SO₄ is tetraamminenickel (II) sulphate.
If the complex is anionic, name of metal ends in –ate followed by oxidation state,
e.g. Cr (chromate), Mn (manganate), Fe (ferrate) etc.
If the co-ordination compound is an acid, name of metal atom ends in –ic,
e.g. H₄[PtCl₆] is hexachloroplatinic (II) acid.
For polynuclear complex word -μ- is added before as name of bridging ligands, e.g.

polynuclear complex

6. For ambident ligands, point of attachment with central metal atom is either indicated by using different names for the ligand, e.g. –SCN⁻ is named as thiocyanato and –NCS⁻ is named as isothiocyanato or isothiocyanato -S- and thiocyanato -N-.

Naming optical isomers: Dextro and laevo rotatory optically active compounds are respectively designated as either by (+) and (–) or d– and ℓ–, e.g. d–K₄[PtCl₆] is potassium (+) or d-hexachloroplatinate.

Example: The IUPAC name of [Cr(H₂O)₄Cl₂]Cl is
(A) tetrahydroidichlorochromium (III) chloride
(B) tetrahydroidichlorochromium (II) chloride
(C) tetrahydroidichlorochromium (I) chloride
(D) None of the above

Solution: (B).

ISOMERISM IN CO-ORDINATION COMPOUNDS

The types of isomerism exhibited by coordination compounds are broadly divided into two types,
1. Structural isomerism
2. Stereoisomerism

Structural Isomerism
Ionization isomerism. When compounds of same molecular formula have different ions in the solution, e.g. [CoBr(NH₃)₅]²⁺ SO₄^2– and [CoSO₄(NH₃)₅]⁺ Br^–.
Hydrate isomerism. When ligand H₂O molecules become water of hydration outside the co-ordination sphere, e.g. [Cr(H₂O)₆]Cl₃ is violet. Its isomer [CrCl(H₂O)₅]Cl₂·H₂O is pale green while [CrCl₂(H₂O)₄]Cl·H₂O is dark green.

Linkage isomerism. Complexes in which ambidentate ligands [e.g. –SCN^– (thiocyanato) and –NCS^– (isothiocyanato)] are present, show linkage isomerism, e.g. [Co(H₂O)₅SCN]²⁺ and [Co(H₂O)₅NCS]²⁺.

Co-ordination isomerism. The complexes in which both cations and anions are complexes and ligands may interchange their positions between the two complex ions, show coordination isomerism, e.g. [Cu(NH₃)₄][PtCl₄] and [PtCl(NH₃)₄][CuCl₄].

Co-ordination position isomerism. It arises in the bridged complexes due to the difference in the attachment of ligands with the metal atoms, e.g.

co-ordination position isomerism

STEREOISOMERISM OR SPACE ISOMERISM

Geometrical isomerism. It is also referred as cis-trans isomerism. When same donor atoms of same ligands (groups) are on adjacent positions, it is called cis isomer while when they are on opposite positions, they are called trans isomers. Such isomerism is usually seen in complexes with co-ordination number 4 (square planar type) and 6 (octahedral).

Co-ordination number 4. Tetrahedral complexes do not show this isomerism. Square planar complexes of type MA₂X₂, MA₂XY, MABY₂, M(AB)₂ and MABXY show cis-trans isomerism.

MA₂X₂ type [Pt(NH₃)₂Cl₂]

MA₂XY [Pt(NH₃)₂ClNO₂]

MABXY [Pt(NH₃)(H₂O)(NO₂)Cl]NO₂

Co-ordination number 6. Octahedral complexes of type M(AA)₂X₂ and M(AA)₂XY show cis-trans isomerism, where (AA) is symmetrical bidentate ligand. Octahedral complexes with all six ligands different (MABCDEF) exist in 15 theoretical isomers.

Optical isomerism. Chiral molecules which do not have plane of symmetry show optical isomerism. Complexes with co-ordination number 4 and 6 having chelating ligands show this kind of isomerism.

Complexes with co-ordination number 4. Square planar complexes having plane of symmetry are optically inactive. Tetrahedral complexes containing unsymmetrical bidentate ligands, i.e. M(AB)₂ type show optical activity.

Complexes with co-ordination number 6. Octahedral complexes of the type [M(AA)X₂Y₂], [M(AA)₂X₂] and [M(AA)₂XY] exists as pair of enantiomers. Trans forms of these complexes are symmetrical and hence optically inactive, cis isomers due to unsymmetry are optically active. Type M(AA)₃ are also optically active, e.g. Cr(en)₃.

Complexes having all six ligands different and complexes with hexadentate ligands such as EDTA are optically active and have d and l isomers.

Bonding in co-ordination compounds

Valence bond theory explains the shapes of complexes and relation between observed magnetic behaviours. The shapes can be explained on the basis of hybridization.

Shapes	Hybridization
Linear	sp
Tetrahedral	sp³
Octahedral	dp³/d²/d²sp³
Square planar	dsp²
Trigonal bipyramidal	dsp³

[Co(CN)₆]³^- has octahedral shape as explained on the basis of hybridization d²sp³.

Bonding in coordination compounds

Complex is diamagnetic due to all paired electrons and absence of any unpaired electrons. Other examples:

Coordination Complexes and Their Properties

S.No.	Complex	Metal ion	Hybridization	No. of unpaired e̅s
1.	[V(H₂O)₆]³⁺	V³⁺	d²sp³	2
2.	[Cu(NH₃)₂]⁺	Cu⁺	sp	0
3.	[Co(CN)₆]^3–	Co³⁺	d²sp³	0
4.	[Pt(NH₃)₂Cl₂]	Pt²⁺	dsp²	0
5.	[CuCl₄]^2–	Cu²⁺	sp³	1

The complexes involving (n-1)d orbitals in hybridization, i.e. dsp² or d²sp³ hybridization are called inner d-orbital complexes. Such complexes are usually diamagnetic. Thus they are called spin paired complexes. Complexes involving outer nd orbtials in sp³d² hybridization are called outer d orbital complexes or high spin complexes.

CRYSTAL FIELD THEORY

This theory assumes that metal ion and the ligands act as point charges and interaction between them is electrostatic. In an isolated gaseous metal atom/ion, the five d-orbitals are degenerate. But in a complex the splitting of energy of d-orbitals takes place. The splitting depends upon the nature of geometry like octahedral, tetrahedral or square planar and the basic strength of ligands.

In case of octahedral complexes, the six ligands approach the central metal atom symmetrically along the three axis. The d-orbitals of central atom which lie along the axes (i.e. d_z² and d_x²–y²), called e.g. orbitals get repelled more strongly by negative ligands which raises their energy. While the inter axial (d_xy, d_yz and d_zx orbitals) are called t_2g orbitals get lowered in energy relative to average energy of d-orbitals (before splitting), as shown below:

coordination compounds

Energy difference between t₂g and eₙ set is measured in Δ₀ or Dq, where, Δ₀ = 10 Dq.

The electrons while filling occupy the lower energy set which stabilizes the system. Filling of electrons in eₙ set destabilizes it. For configuration d¹, d² and d³ lower set is filled first in accordance with Hund’s rule. For d⁴ configuration, the fourth electron goes to eₙ set, if splitting is less and is done by weak ligands like H₂O, F⁻, Cl⁻, Br⁻, I⁻. Hence, all electrons are unpaired giving high spin complexes. But for strong field ligands the splitting is much more and Δ₀ > P, so pairing takes place and fourth electron again enters t₂g set. [P = Pairing energy]

Pairing energy

In Tetrahedral Complexes

The ligands are present between the axes due to which the inter axial orbitals experience more repulsion and have higher energy. Hence, reverse of octahedral type splitting takes place.

coordination compounds

Δₜ ≈ ⁴⁄₉ Δ₀

Tetrahedral complexes have four ligands hence splitting is less due to which only high spin tetrahedral complexes are formed.

Conditions for Stability of Complexes

Greater the charge on central atom, greater is the splitting, greater is the stability.
More the basic strength of ligands, more is the splitting, greater is the stability.
More the basic strength of ligands, more is the stability.

ORGANOMETALLIC COMPOUNDS

Compounds containing at least one carbon–metal bond are called organometallic compounds.

σ Bonded Complexes

σ bonded complexes have σ bonds between carbon and metal, e.g. Pb(C₂H₅)₄, CH₃MgBr etc.

π Bonded Complexes

In π bonded complexes π electrons of ligands are involved in metal–carbon bond formation, e.g. Zeise’s salt K[PtCl₃(η²—C₂H₄)], where η² shows two carbon atoms are attached to central atom of the complex via π electron donation.

Metal Carbonyls

In metal carbonyls both σ and π bonding is involved.

A σ bond between metal and carbon is formed when vacant orbitals of metal atom overlap with an orbital on carbon atom of (CO) molecule containing a lone pair of electrons.

π bond is formed when filled orbital of metal atom overlaps with vacant antibonding. π* orbital of carbon atom of carbon monoxide. This overlap is called back donation of electrons by metal atom to carbon or π-back bonding. The give and take relationship of electrons is called synergic effect. Similarly, in olefinic complexes, the bonding π- orbital electrons are donated to empty orbitals of the metal atom and at the same time back bonding occurs from filled orbitals of the metal atom to the antibonding π-orbitals of the olefin.

CHAPTER AT A GLANCE

All transition elements are d–block elements but all d–block elements are not transition elements. To justify this statements we take the examples of Zn. Cd and Hg which are last numbers of each d–block series. These elements are not called transition elements because they have (n − 1)d¹⁰ns² type of completely filled electronic configuration and do not shoe the characteristic properties of transition elements except complex formation.
First member of each transition series i.e., Sc, Y, La and Ac do not show variable valency. They show only +3 oxidation state.

3. Fe, Co and Ni are called ferrous metals; Ru, Rh, Pd, Os, Ir and Pt are called platinum metlas whereas Cu, Ag and Au are called coinage metals.

4. Cu²⁺ ion is more stable than Cu⁺ ion due to extensive hydration as it has a higher charge and lesser size as compared to Cu⁺ (d¹⁰ configuration) ion.

5. Both Au and Pt are inert and noble metals but they dissolve in aqua regia (3 parts conc. HCl + 1 part conc. HNO₃) due to the formation of H₂PtCl₆ and HAuCl₄ respectively.

6. TiCl₄ and TiO₂ are used in smoke screens; tantalum is used in surgical venals and analytical weights; chromium is used in stainless steel and chrome plating; molybdenum is used in X-rays tubes; platinum is used in resistance thermometers; zinc is used in glavanising iron sheets while cadmium is used for making joints in jewellary. TiO₂ is also used as white pigment in paints, Cerium is used as a scavenger of oxygen and sulphur in many metals.

7. Finely reduce form of Pt in the form of velvety black powder is called platinum black.

8. There are many compounds of transition metals which are used as catalysts. For example,

Adam’s catalyst - Pt.PtO

Brown’s catalyst - Nickel boride

Zeigler-Natta catalyst - TiCl₄ + (C₂H₅)₃Al

Wilkinson’s catalyst - [Ph₃P]₃RhCl

9. There are many compound of transition elements which are used as reagents in laboratory/industry. For example,

Baeyer’s reagent - Dilute alkaline solution of KMnO₄

Schweitzer reagent - [Cu(NH₃)₄]SO₄

Nessler’s reagent - Alkaline solution of K₂[HgI₄]

Benedict’s solution - CuSO₄ solution + sodium citrate + Na₂CO₃

Fenton’s reagent - FeSO₄ + H₂O₂

Etard’s reagent - CrO₂Cl₂

Bordeaux mixture - CuSO₄ solution + lime

Milon’s reagent - Solution of mercuric and mercurous nitrate

10. In Pt complexes 6-coordination Pt atom is generally tetravalent and 4-coordinated Pt atom is divalent. e.g., in [Pt(NH₃)₄][PtCl₆] coordination compound, first Pt atom is divalent and second Pt atom is tetravalent.

11. In square planar complexes, positions 1, 2; 2, 3; 3, 4 and 1, 4 are cis while 1, 3 and 2, 4 are trans with respect to each other. In octahedral complexes, positions 1, 2; 1, 3; 1, 4 and 1, 5 are cis while 1, 6; 2, 4 and 3, 5 are trans with respect to each other.

coordination compound

12. Effective atomic number (EAN) rule. Sidgwick proposed effective atomic number (EAN) to explain the stability of metal in a complex. EAN is defined as the total number of electrons on the metal atom or ion after the complex formation which should be equal to that of the next higher noble gas.

If Z is the atomic number, (O.N.) is the oxidation number and (C.N.) is the co-ordination number of the central metal atom or ion in the complex then

EAN = Z - (O.N.) + 2 ´ (C.N.)

13. Labile complex. It is that complex in which ligands can be readily replaced by other ligands.

14. Co-ordination number of the central metal atom is also called ligancy of the metal atom.

15. Prefect or penetrating complexes. These are the compounds in which complex ion is fairly stable and is either not dissociated or feebly dissociated in solution.

e.g. K₄[Fe(CN)₆] → 4K⁺ + [Fe(CN)₆]⁴⁻ ⇌ Fe²⁺ + 6CN⁻ (feebly dissociated)

16. Imperfect or normal complexes. These are the compounds in which complex ion is less stable and is reversibly dissociated to give enough simple ions.
e.g. K₂[Cd(CN)₄] → 2K⁺ + [Cd(CN)₄]²⁻ ⇌ Cd²⁺ + 4CN⁻ (appreciably dissociated)

17. The magnetic criteria of the bond type explains that the geometry of the coordination entity can be predicted if its magnetic behaviour is known. Thus, the diamagnetic [Ni(CN)₄]²⁻ is square planar while the paramagnetic [NiCl₄]²⁻ is tetrahedral.

Solved Example

1. A metal gives two chlorides A and B. A gives black precipitate with NH4OH and B gives white. With KI, B gives a red precipitate which is soluble in excess of KI. A and B are respectively
(A) HgCl₂ and Hg₂Cl₂
(B) Hg₂Cl₂ and HgCl₂
(C) HgCl₂ and ZnCl₂
(D) ZnCl₂ and HgCl₂

Sol. (B). Hg₂Cl₂ + 2NH₄OH → Hg + NH₂HgCl + NH₄Cl + 2H₂O (A) black ppt.
HgCl₂ + 2NH₄OH → HgNH₂Cl + NH₄Cl + 2H₂O (B) white ppt.
HgCl₂ + 2KI → HgI₂ + 2KCl (B) red ppt.
HgI₂ + 2KI → K₂[HgI₄] (soluble)

2. A blue colouration is not obtained when
(A) ammonium hydroxide dissolves in copper sulphate
(B) copper sulphate solution reacts with K₄[Fe(CN)₆]
(C) ferric chloride reacts with sodium ferrocyanide
(D) anhydrous CuSO₄ is dissolved in water

Sol. (B).
2CuSO₄ + K₄[Fe(CN)₆] → Cu₂[Fe(CN)₆] + 2K₂SO₄ (chocolate ppt.)

3. Which of the following ions will finally give a black precipitate with Ag⁺ ion?

(A) SO₃²⁻

(B) Br⁻

(D) S₂O₃²⁻

Sol. (D).
2Ag⁺ + S₂O₃²⁻ → Ag₂S₂O₃, white ppt. which readily changes to yellow, orange, brown and finally black due to the formation of silver sulphide.
Ag₂S₂O₃ + H₂O → H₂SO₃ + Ag₂S (black)

4. The compound which gives oxygen on moderate heating is

(A) zinc oxide

(B) mercuric oxide

(D) ferric oxide

Sol. (B).
2Hg₂O —heat→ 4Hg + O₂

5. Which one of the following has the least magnetic moment?

(A) Cu²⁺

(B) Ni²⁺

(D) Fe²⁺

Sol. (A).
Cu²⁺ = 3d⁹ (1 unpaired electron); Ni²⁺ = 3d⁸ (2 unpaired electrons); Co²⁺ = 3d⁷ (3 unpaired electrons) and Fe²⁺ = 3d⁶ (4 unpaired electrons). Hence, Cu²⁺ shows least magnetic moment.

Frequently Asked Questions

D-block elements — the transition metals — stand out in the periodic table because of their remarkable ability to form coordination complexes, and this is rooted in their unique electron configuration, oxidation behavior, and bonding tendencies.

1. Partially Filled d-Orbitals
The hallmark of transition metals is their partially filled (n–1)d orbitals. These orbitals act like “vacant guest rooms” ready to accommodate lone pairs from ligands (ions or molecules capable of donating electrons). This allows the metal to form coordinate covalent bonds easily. For example, copper(II) forms [Cu(NH₃)₄]²⁺ with ammonia, producing an intense deep-blue color. This property is exploited in qualitative inorganic analysis to confirm the presence of Cu²⁺.

2. Variable Oxidation States
Unlike main group metals, transition elements can exhibit multiple oxidation states because their (n–1)d and ns orbitals are close in energy. This means they can lose different numbers of electrons depending on the ligand and reaction conditions. For instance, iron can exist as Fe²⁺ or Fe³⁺, each with a distinct ability to bind different ligands and form complexes like [Fe(CN)₆]³⁻ or [Fe(CN)₆]⁴⁻. This flexibility broadens the range of stable complexes they can form.

3. High Charge Density and Small Size
Most d-block ions are small yet highly charged (e.g., Fe³⁺, Cr³⁺). This results in a strong polarizing power, attracting electron-rich ligands and creating strong metal-ligand bonds. The smaller the ionic radius and the higher the charge, the stronger the interaction — which explains why Cr³⁺ forms extremely stable complexes with ligands like NH₃ and H₂O.

4. Ligand Field Stabilization Energy (LFSE)
From a more advanced perspective, d-orbital splitting in the presence of ligands provides additional stability. Ligands cause the degenerate d-orbitals to split into two energy levels, and the filling of these orbitals in specific patterns can make certain geometries (like octahedral or square planar) particularly stable. This theory explains why some complexes are more favored than others.

5. Real-Life Relevance
Complex formation is not just a laboratory curiosity — it’s biologically and industrially essential. Hemoglobin, the oxygen carrier in our blood, is an iron(II) complex with a porphyrin ligand. In industry, nickel complexes catalyze hydrogenation reactions, and platinum complexes like cisplatin are used in cancer therapy.

Exam Tip:
For NEET/JEE, remember these four pillars:

Partially filled d-orbitals for accepting electron pairs.
Variable oxidation states for multiple bonding possibilities.
High charge density attracting ligands strongly.
LFSE providing extra stability.

In Ni(CO)₄, nickel is in the zero oxidation state — and this can be understood step-by-step:

Nickel’s atomic number is 28, so its ground-state configuration is [Ar] 3d⁸ 4s².
In Ni(CO)₄, CO is a neutral ligand (it does not contribute any net charge).
The total charge on the complex is zero. Since ligands are neutral, the metal’s oxidation state must also be zero to maintain neutrality.
Therefore, Ni remains in oxidation state 0.

From a bonding perspective, CO donates electron density via its lone pair on carbon to the vacant orbitals of Ni. At the same time, Ni can back-donate electron density into CO’s empty π* orbitals — a phenomenon known as π-back bonding. This makes Ni(CO)₄ an 18-electron complex (Ni has 10 valence electrons in 3d⁸ 4s², and CO donates 8 electrons total), which is exceptionally stable.

This stability explains why Ni(CO)₄ is used in the Mond process for purifying nickel: impure nickel reacts with CO to form Ni(CO)₄, which is volatile and can be decomposed to yield pure nickel.

The name follows IUPAC rules:

Cation is named first → Potassium.
The complex anion [CoF₆]³⁻ is named next. Fluoride is the ligand (fluoro), and cobalt’s oxidation state is determined:
- K₃ means three K⁺ ions = +3 total charge.
- The complex anion is –3 total, so cobalt must be in +3 oxidation state.
Name: Potassium hexafluorocobaltate(III).

If you think of your Class 12 Chemistry syllabus as a series of puzzle pieces, D-block elements and Coordination compounds are two pieces that fit very closely together — but one of them makes the other much easier to understand.

Here’s why:
D-block elements give you the foundation: their position in the periodic table, electronic configurations, variable oxidation states, magnetic properties, and color — all of which directly explain why and how they form coordination compounds. If you skip straight to Coordination Compounds, you’ll still learn about ligands, coordination numbers, and IUPAC naming, but you’ll be missing the “why these compounds exist in the first place” part.

For example, the fact that Fe³⁺ forms a stable [Fe(CN)₆]³⁻ complex becomes far more intuitive once you already understand that Fe³⁺ has a small ionic radius, high charge density, and partially filled d-orbitals. Without this base, you’ll just be memorizing facts instead of understanding concepts.

Best practice for students (NEET/JEE/Boards)

Step 1: Study D-block elements first — focus on general characteristics, trends across the series, and exceptions.
Step 2: Move to Coordination Compounds — here you’ll see those properties in action.
Step 3: Link back and forth — as you learn ligand field theory, recall what you learned about oxidation states, and you’ll find it easier to answer application-based questions.

Exam insight: Coordination Compounds often carry 5–8 marks in Board Exams and multiple direct MCQs in NEET/JEE. Understanding the D-block first can boost your score because you’ll handle conceptual questions faster.

Think of D and F block chemistry as Level 3 in a video game — you need to unlock Levels 1 and 2 first. If your basics aren’t solid, here’s what you should cover before tackling them:

1. Periodic Table Basics
Understand how elements are arranged and what defines blocks (s, p, d, f). This helps you grasp why transition metals sit in the middle, why their properties are “in-between” metals and nonmetals, and why they’re called transition elements.

2. Atomic Structure & Electron Configuration
D-block chemistry revolves around the filling and emptying of (n–1)d and ns orbitals. You need to know:

How to write electron configurations.
The role of Hund’s rule and Aufbau principle.
Why Cr and Cu have unusual configurations.

3. Periodic Trends
Ionic radius, ionization energy, electronegativity trends — these control oxidation states and complex stability.

4. Chemical Bonding
Basics of covalent, ionic, and coordinate covalent bonding — especially how lone pairs work.

5. Oxidation Numbers
Since transition metals have multiple oxidation states, knowing how to assign oxidation numbers is a must.

Once you’ve mastered these, the D and F block chapters will feel logical rather than overwhelming.

Exam tip: If you’re weak in basics, dedicate 1–2 days to just brushing up on Periodic Table trends and electron configurations — these two alone cover half the conceptual weight in D and F block questions.

Technically, yes — but it’s not ideal. Skipping S and P block means missing the comparative framework. Many D-block properties make sense only when you contrast them with S and P blocks:

S-block metals have fixed oxidation states; D-block metals have variable oxidation states.
P-block elements form covalent compounds; D-block elements tend to form colored complexes.

If you skip them, you’ll understand facts but not patterns. That’s like reading the middle of a story without knowing the beginning.

Recommendation:

If you’re short on time before exams, you can start D-block, but skim through the S and P block general properties chapters.
At the very least, know the trends in metallic character, atomic size, and electronegativity from S to P to D block.

Before diving into Coordination Compounds, be comfortable with:

D-block general properties (oxidation states, ionic sizes, magnetic behavior).
Chemical bonding basics (especially coordinate bonds).
Nomenclature rules — since naming coordination compounds follows strict IUPAC patterns.
Hybridization and VSEPR theory — needed to predict geometries of complexes.

NEET is a speed-and-accuracy game, and the D & F block elements chapter is a low-weightage but high-return topic. While it may only contribute 3–4 questions, these are often direct and fact-based — meaning you can secure full marks if your revision method is strategic.

Step 1: Understand, Don’t Just Memorize Trends
Start with the general properties:

Electronic configuration of each series (3d, 4d, 5d for D-block; 4f, 5f for F-block).
Trends in atomic/ionic radii, ionization enthalpy, oxidation states, and magnetic properties.
Once you understand why properties change across a period or down a group, memorizing becomes 10x easier. For example: Atomic radius decreases slightly across the D-block because of increased nuclear charge and ineffective shielding by d-electrons.

Step 2: Focus on High-Yield Topics for NEET

Color and magnetic properties of complexes (Ligand Field Theory basics).
Biological importance of metals (e.g., hemoglobin, chlorophyll).
Important compounds like KMnO₄, K₂Cr₂O₇ — preparation, properties, and uses.
Lanthanoid contraction and its consequences.

Step 3: Use Active Recall
Instead of passively reading, try question-answer cards. Example:
Q: Why is Sc³⁺ colorless?
A: No unpaired d-electrons → no d–d transitions → colorless.

Step 4: Integrate Coordination Chemistry Concepts
Since many NEET questions blend D-block with Coordination Compounds, study them together. Example: “Which ion in the D-block forms a square planar complex with CN⁻ ligands?” — This needs both D-block knowledge and hybridization concepts.

Step 5: Mock Tests & NCERT Mastery
90% of NEET inorganic chemistry comes directly from NCERT lines — so underline every fact in the NCERT chapter. Then, attempt 30–40 MCQs daily from past NEET papers and similar mock tests.

NEET Exam Hack for D & F Block:
The quickest marks in this topic come from:

Oxidation state patterns.
Lanthanoid contraction facts.
Color/magnetism of complexes.
Important reactions of KMnO₄ & K₂Cr₂O₇.

Board exams are not just about knowing they’re about writing answers exactly the way the examiner expects. In D & F block, concept clarity + crisp presentation is the key.

1. Prioritize NCERT Line-by-Line Reading
Boards rarely go beyond NCERT for inorganic chemistry. Mark every trend, table, and reaction mechanism. Pay special attention to examples and exceptions (like Cu⁺ being unstable in aqueous solution).

2. Use Structured Notes
For each series (3d, 4d, 5d, lanthanoids, actinoids), prepare a one-page summary including:

Electronic configuration.
Oxidation states and stability trends.
Magnetic properties.
Color and ligand preferences.

3. Practice Reactions with Balanced Equations
In exams, incomplete equations lose marks. For example:
2MnO₄⁻ + 10I⁻ + 16H⁺→ 2Mn²⁺ + 5I₂ + 8H₂O.
Write them repeatedly to lock them in memory.

4. Visual Memory Tools
Draw periodic table segments highlighting only D & F block elements. Use color codes for oxidation states and magnetic properties.

5. Previous Year Questions
Boards often repeat questions, sometimes verbatim. Practice at least 10 years of previous papers for D & F block.

JEE Mains requires both NCERT mastery and application ability. Questions can be direct (fact-based) or indirect (concept + logic).

1. Focus on Conceptual Understanding
Why Ti³⁺ is colored but Sc³⁺ is not; why Cr²⁺ is reducing but Mn³⁺ is oxidizing — these “why” questions are JEE favorites.

2. Link with Coordination Chemistry
Many JEE D-block questions are disguised Coordination Chemistry problems (magnetic moment, geometry prediction).

3. Practice Mixed MCQs
Don’t isolate topics — practice questions where you identify oxidation state + predict hybridization + magnetic property all in one go.

4. Solve PYQs from JEE Mains
Most D-block JEE questions are repeated with minor tweaks every few years.

For NEET/JEE/Boards, focus on:

Definition of ligand, coordination number, coordination entity.
IUPAC nomenclature rules.
Types of ligands (monodentate, bidentate, polydentate, ambidentate).
Isomerism (structural and stereoisomerism).
Bonding theories (VBT and CFT).
Applications (medicine, industry, biology).

In Class 12 Chemistry, the d-block elements are taught as part of the transition metals section of the periodic table. They occupy the central part of the periodic table — Groups 3 to 12 — and are characterized by having their d-orbitals partially filled in at least one common oxidation state.

Classification by Periods and Series
The d-block elements are divided into four series based on the principal quantum number of the (n–1)d orbitals being filled:

3d series (First Transition Series) – Period 4, from Scandium (Sc) to Zinc (Zn).
4d series (Second Transition Series) – Period 5, from Yttrium (Y) to Cadmium (Cd).
5d series (Third Transition Series) – Period 6, from Hafnium (Hf) to Mercury (Hg).
6d series (Fourth Transition Series) – Period 7, starting from Rutherfordium (Rf). (Only some are well-studied due to radioactivity.)

Groups and Special Members

Group 3 elements: Sc, Y, La/Ac (sometimes debated due to periodic table arrangements).
Group 11: Cu, Ag, Au — known as coinage metals.
Group 12: Zn, Cd, Hg — often considered “pseudo-transition” metals because their d-orbitals are completely filled in the most stable oxidation state.

Why Classification Matters for Exams
This classification helps in spotting trends — like magnetic properties, color, oxidation states — because these trends repeat within a series. For example, all members of Group 6 (Cr, Mo, W) tend to have high oxidation states (+6).

There are 10 elements in each row (series) of the d-block because the d-subshell can hold a maximum of 10 electrons. The general electronic filling for these elements is:

(n–1)d¹ to (n–1)d¹⁰
With ns¹ or ns² as outermost electrons.

Since each d-block series corresponds to filling one set of d-orbitals, there are 10 positions in that block per period.

Total Count in the Periodic Table:

3d series: 10 elements.
4d series: 10 elements.
5d series: 10 elements.
6d series: up to 10 elements (though superheavy elements are unstable).
This makes roughly 40 d-block elements (including radioactive ones).

The term “transition elements” comes from the fact that these elements represent a transition in properties between the electropositive s-block metals and the electronegative p-block nonmetals in the periodic table.

Key reasons for the name:

Electronic Transition: The filling of d-orbitals marks a gradual change in electron configuration between blocks.
Property Transition: Metallic character, oxidation states, and bonding types change gradually across the series.
Chemical Versatility: They can show both metallic properties (conductivity, malleability) and nonmetal-like behavior (forming covalent bonds).

An example: Titanium sits between calcium (highly reactive s-block metal) and gallium (a p-block element with more nonmetallic tendencies) — and shows mixed characteristics.

Textbook Definition:
A transition element is defined as “an element that has a partially filled d-orbital in its ground state or in any one of its common oxidation states.”

The name “d-block” simply means the block of the periodic table where the last electron enters a d-orbital. The “d” refers to the type of atomic orbital (d-orbitals have l = 2 in quantum mechanics).

Quick facts:

Position: Groups 3–12, between s-block and p-block.
Outer electron configuration: (n–1)d¹–¹⁰ ns¹–².
Signature properties: Variable oxidation states, colored compounds, catalytic behavior, tendency to form complexes.

In Real Life:
The properties of d-block metals explain why they dominate industries:

Iron in construction.
Platinum in catalytic converters.
Copper in electrical wiring.
Titanium in aerospace.

S.no	Formulas List
1.	Liquid Solutions
2.	Electrochemistry
3.	Chemical Kinetics
4.	Ores and Metallurge
5.	P-Block Elements
6.	D-Block and Co-ordination Compounds
7.	Chemistry of Heavy Metals
8.	Alkyl and Aryl Halides
9.	Alcohol phenol and ether
10.	Aldehyde and Ketone

D-Block and Co-ordination Compounds

Solved Example

Frequently Asked Questions

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