Properties of d-Block Elements

Introduction

The unique properties of d-block elements make them indispensable in industry, biology, and technology. From the colors in gemstones to the catalysts in your car, transition metals are everywhere!

Variable Oxidation States

The most characteristic property of transition elements!

Why Variable Oxidation States?

Reasons:

1. Similar energy of (n-1)d and ns orbitals
2. Small energy difference between successive oxidation states
3. Both ns and (n-1)d electrons can participate in bonding

Oxidation State Trends

graph TD
    A[Variable OS] --> B[Increases up to Mn]
    A --> C[Decreases after Mn]
    B --> B1[Max OS = Group Number]
    C --> C1[Pairing Energy Effect]

Complete Oxidation State Chart (3d series):

Stability of Oxidation States

General Rules:

1. Lower OS (≤+3): Generally stable in solution
2. Higher OS (≥+4): Strong oxidizing agents
3. Intermediate OS: May disproportionate

Stability Patterns:

Disproportionation Reactions

Some oxidation states are unstable and undergo disproportionation:

Examples:

1. Mn³⁺ disproportionation:

   $$2Mn^{3+} \rightarrow Mn^{2+} + Mn^{4+}$$

   (Mn³⁺ is unstable in solution)

2. Cu⁺ disproportionation:

   $$2Cu^+ \rightarrow Cu^{2+} + Cu$$

   (Cu⁺ is unstable in aqueous solution)

3. Mn⁶⁺ disproportionation:

   $$3MnO_4^{2-} + 4H^+ \rightarrow 2MnO_4^- + MnO_2 + 2H_2O$$

Color in Transition Metal Compounds

Origin of Color

d-d Transitions in the visible region!

graph TB
    A[White Light] --> B[Compound]
    B --> C[Absorbs certain wavelength]
    B --> D[Transmits complementary color]
    C --> E[Electron jumps between split d-orbitals]
    D --> F[Observed Color]

Crystal Field Theory Basics

When ligands approach, d-orbitals split into different energy levels:

Octahedral Splitting:

• Lower energy: t₂g (dxy, dyz, dzx)
• Higher energy: eg (dz², dx²-y²)

Color depends on:

1. Number of d-electrons
2. Magnitude of splitting (Δ)
3. Type of ligands
4. Geometry

Absorption and Color Wheel

d-electron Count and Color

Rule: Compounds with d⁰ or d¹⁰ configuration are colorless (no d-d transitions possible)

*Color due to charge transfer, not d-d transitions

Interactive Demo: Visualize d-Block Transition Metal Properties

Explore oxidation states, color transitions, and periodic trends across the d-block.

Charge Transfer Complexes

Some colored compounds have d⁰ or d¹⁰ but still show color due to charge transfer:

Examples:

1. KMnO₄ (purple): Mn⁷⁺ is d⁰

   • Charge transfer: O²⁻ → Mn⁷⁺

2. K₂Cr₂O₇ (orange): Cr⁶⁺ is d⁰

   • Charge transfer: O²⁻ → Cr⁶⁺

3. Cu₂O (red): Cu⁺ is d¹⁰

   • Charge transfer: Cu⁺ → Cu²⁺

Magnetic Properties

Types of Magnetism

graph TD
    A[Magnetic Behavior] --> B[Diamagnetic]
    A --> C[Paramagnetic]
    A --> D[Ferromagnetic]
    B --> B1[No unpaired e⁻]
    C --> C1[Unpaired e⁻ present]
    D --> D1[Aligned magnetic domains]

Paramagnetism in Transition Metals

Most transition metal compounds are paramagnetic due to unpaired d-electrons.

Magnetic Moment Formula (Spin-only):

where:

• n = number of unpaired electrons
• BM = Bohr Magneton

Calculating Magnetic Moment

Step-by-step:

1. Determine ion’s electronic configuration
2. Count unpaired electrons
3. Apply formula: μ = √n(n+2)

Examples:

High Spin vs Low Spin Complexes

Magnetic moment depends on ligand field strength:

Weak Field Ligands (I⁻, Br⁻, Cl⁻, F⁻, OH⁻):

• Small Δ
• High spin (maximum unpaired electrons)
• Example: [Fe(H₂O)₆]²⁺ has 4 unpaired e⁻

Strong Field Ligands (CN⁻, CO, NO₂⁻):

• Large Δ
• Low spin (minimum unpaired electrons)
• Example: [Fe(CN)₆]⁴⁻ has 0 unpaired e⁻

Ferromagnetism

Permanent magnetism shown by Fe, Co, Ni due to:

• Aligned magnetic domains
• Persists even after removing external field

Curie Temperature: Temperature above which ferromagnetic → paramagnetic

• Fe: 1043 K
• Co: 1394 K
• Ni: 631 K

Catalytic Properties

Transition metals are excellent catalysts in industrial and biological processes.

Why Good Catalysts?

Reasons:

1. Variable oxidation states

   • Can donate/accept electrons easily
   • Form reaction intermediates

2. Form complexes

   • Bring reactants together
   • Lower activation energy

3. Large surface area

   • Provide adsorption sites (heterogeneous catalysis)

4. Partially filled d-orbitals

   • Facilitate bonding with reactants

Homogeneous Catalysis

Catalyst in the same phase as reactants.

Examples:

1. Contact Process (SO₂ → SO₃):

   $$2SO_2 + O_2 \xrightarrow{V_2O_5} 2SO_3$$

   Mechanism:

   $$V_2O_5 \rightarrow V_2O_4 + \frac{1}{2}O_2$$ $$V_2O_4 + SO_2 \rightarrow V_2O_5 + SO_3$$

2. Oxidation of CO:

   $$2CO + O_2 \xrightarrow{[Co(OAc)_2]} 2CO_2$$

3. Wilkinson’s Catalyst:

   $$RCH=CH_2 + H_2 \xrightarrow{[RhCl(PPh_3)_3]} RCH_2CH_3$$

Heterogeneous Catalysis

Catalyst in different phase from reactants.

Industrial Examples:

Mechanism (Hydrogenation):

1. Adsorption: Reactants bind to metal surface
2. Activation: Bonds weaken
3. Reaction: Products form on surface
4. Desorption: Products leave surface

graph LR
    A[H₂ + C=C] --> B[Adsorption on Ni]
    B --> C[H-H and C=C bonds weaken]
    C --> D[H atoms add to C]
    D --> E[C-C-H product desorbs]

Biological Catalysis

Metalloenzymes containing transition metals:

Formation of Interstitial Compounds

Interstitial compounds form when small atoms (H, B, C, N) occupy interstitial spaces in the metal lattice.

Characteristics

Properties:

1. Very hard and rigid
2. High melting points
3. Chemically inert
4. Retain metallic conductivity
5. Non-stoichiometric (variable composition)

Examples:

• TiC, TiH₁.₇, VH₀.₅₆, Fe₃H, Mn₄N, Fe₃C (cementite)

Steel Formation

Steel = Iron + Carbon (interstitial compound)

The carbon atoms fit into interstitial spaces, making steel harder than pure iron!

Alloy Formation

Transition metals readily form alloys due to:

• Similar atomic radii
• Similar crystal structures
• Similar metallic bonding

Types:

1. Substitutional Alloys: Metal atoms replace each other

   • Brass (Cu + Zn)
   • Bronze (Cu + Sn)

2. Interstitial Alloys: Small atoms in gaps

   • Steel (Fe + C)

Common Alloys:

Complex Formation

Transition metals form coordination complexes readily.

Why?

1. Small size + high charge → high charge density
2. Vacant d-orbitals → can accept electron pairs
3. Variable oxidation states → flexible bonding

Examples

Common Complexes:

Stability: Related to:

• Charge on metal ion (higher charge = more stable)
• Size of metal ion (smaller = more stable)
• Nature of ligand (chelating > monodentate)

Common JEE Mistakes

1. Assuming d⁰ compounds are always colorless

   • Wrong: KMnO₄ (Mn⁷⁺, d⁰) is purple due to charge transfer

2. Forgetting to consider ligand field in magnetic moment

   • [Fe(CN)₆]⁴⁻ is diamagnetic (low spin)
   • [Fe(H₂O)₆]²⁺ is paramagnetic (high spin)

3. Maximum oxidation state confusion

   • Mn shows +7 (uses all 7 electrons)
   • Fe maximum is +6 (not +8)

4. Cu⁺ stability

   • Unstable in aqueous solution (disproportionates)
   • Stable in solid compounds like CuI

5. Magnetic moment calculation errors

   • Always find the ION’s configuration first
   • Fe²⁺ is 3d⁶ (not 3d⁶4s²)

Practice Problems

Level 1: Basic Understanding

1. Why are most transition metal compounds colored?

2. Calculate magnetic moment of:

   • Fe²⁺
   • Co³⁺
   • Cu⁺

3. Why is Zn²⁺ diamagnetic while Cu²⁺ is paramagnetic?

Level 2: Application

4. Compare:

   • Color of Ti⁴⁺ and Ti³⁺ compounds
   • Stability of Mn²⁺ and Mn³⁺
   • Magnetic nature of [Fe(CN)₆]⁴⁻ and [Fe(H₂O)₆]²⁺

5. Explain why:

   • KMnO₄ is purple though Mn⁷⁺ is d⁰
   • Fe is used in Haber process as catalyst
   • Interstitial compounds are very hard

6. A complex of Co³⁺ has magnetic moment 4.90 BM. Is it high spin or low spin? Give structure.

Level 3: JEE Advanced

7. The magnetic moment of [Mn(CN)₆]⁴⁻ is 2.8 BM. The geometry and number of unpaired electrons are:

   • (a) Octahedral, 2
   • (b) Tetrahedral, 3
   • (c) Square planar, 2
   • (d) Octahedral, 4

8. Which is most stable in aqueous solution?

   • (a) Cu⁺
   • (b) Fe³⁺
   • (c) Mn³⁺
   • (d) Cr²⁺

9. A transition metal ion M²⁺ has configuration 3d⁵ in free state. Its magnetic moment in [M(CN)₆]⁴⁻ is 1.73 BM. Identify the metal.

10. Assertion (A): CrO₄²⁻ is yellow while Cr₂O₇²⁻ is orange. Reason (R): CrO₄²⁻ and Cr₂O₇²⁻ have different geometries.

    • (a) Both A and R true, R explains A
    • (b) Both true, R doesn’t explain A
    • (c) A true, R false
    • (d) Both false

Memory Tricks

“CCOOM” for Properties

Key Properties:

• Color formation
• Catalytic activity
• Oxidation state variability
• Octahedral complexes (common)
• Magnetic behavior

Magnetic Moment Quick Values

Color Memory

“Silly Tigers Very Carefully Make Friends Carefully, Not Carelessly, Zipping!”

• Sc³⁺ = Colorless
• Ti³⁺ = Purple
• V³⁺ = Green
• Cr³⁺ = Green
• Mn²⁺ = Pink
• Fe²⁺ = Green
• Fe³⁺ = Yellow (Careful!)
• Co²⁺ = Pink
• Ni²⁺ = Green
• Cu²⁺ = Blue
• Zn²⁺ = Colorless

Related Topics

Within d-f Block Elements

• Transition Elements - General characteristics
• Important Compounds - K₂Cr₂O₇, KMnO₄
• Lanthanoids - f-block properties
• Actinoids - Radioactive elements

Other Chemistry Topics

• Coordination Compounds - CFT, VBT
• Chemical Bonding - Molecular orbital theory
• Atomic Structure - Electronic configuration

Cross-Subject Connections

• Magnetism - Ferro-, para-, diamagnetism
• Electrochemistry - Oxidation states