Course Topics

Introduction to Transition Elements

Definition: “The elements which have partially filled d or f-orbitals either in their atomic states or in any other commonly occurring oxidation states are called transition elements.” They are called d-block or f-block elements.

Typical Transition Elements

  • Have partially filled d-orbitals
  • Show characteristic properties
  • Example: Ti, V, Cr, Mn, Fe, Co, Ni

Non-typical Transition Elements

  • Group II-B (Zn, Cd, Hg)
  • Group III-B (Sc, Y, La)
  • d-orbitals completely filled
CRITICAL CONCEPT:
  • d-block elements: Called outer transition elements (3d, 4d, 5d series)
  • f-block elements: Called inner transition elements (Lanthanides & Actinides)
Memory Tip

Transition elements = Partially filled d/f orbitals = Variable oxidation states + Colors + Catalytic activity

Series of Transition Elements

d-block elements consist of three series of ten elements each:

Series Elements Atomic Numbers
3d-series Scandium (Sc) to Zinc (Zn) 21 to 30
4d-series Yttrium (Y) to Cadmium (Cd) 39 to 48
5d-series Lanthanum (La) to Mercury (Hg) 57 to 80

General outermost configurations:

d-block: (n-1)d1-10 ns1-2
f-block: (n-1)d10 (n-2)f1-14 ns1-2
Memory Tip

Remember the series: 3d (Sc-Zn), 4d (Y-Cd), 5d (La-Hg) – each has 10 elements!

General Characteristics

Characteristic Description
Metallic Nature All are metallic with metallic bonds
Catalytic Activity Most act as catalysts (Ti, Cr, Fe, Ni, Cu, etc.)
Hardness & Conductivity Hard, strong metals with high m.p. & b.p., good conductors
Alloy Formation Form alloys with each other and other elements
Variable Oxidation States Show multiple oxidation states (except few)
Colored Compounds Ions and compounds are colored (due to d-d transitions)
CRITICAL CONCEPT: The unique properties of transition elements arise from:
  1. Partially filled d-orbitals
  2. Small size of atoms
  3. High nuclear charge
  4. Ability to form complexes
Memory Tip

Transition metals: Hard, Colored, Catalytic, Variable oxidation states, Alloy-forming

Electronic Configuration

Key Point: Transition elements have partially filled ‘d’ or ‘f’ sub-shells in atomic state or in any of their commonly occurring oxidation states.

3d-block Elements Electronic Configuration 4d-block Elements Electronic Configuration
Sc (21) [Ar] 3d¹ 4s² Y (39) [Kr] 4d¹ 5s²
Ti (22) [Ar] 3d² 4s² Zr (40) [Kr] 4d² 5s²
V (23) [Ar] 3d³ 4s² Nb (41) [Kr] 4d⁴ 5s¹
Cr (24) [Ar] 3d⁵ 4s¹ Mo (42) [Kr] 4d⁵ 5s¹
Mn (25) [Ar] 3d⁵ 4s² Tc (43) [Kr] 4d⁵ 5s²
Fe (26) [Ar] 3d⁶ 4s² Ru (44) [Kr] 4d⁷ 5s¹
Memory Tip

Exceptions: Cr & Cu have half-filled/full d-subshell stability: Cr = 3d⁵4s¹, Cu = 3d¹⁰4s¹!

Variable Oxidation States

Reason: Transition elements have d-electrons in addition to s-electrons for bond formation. The energies of (n-1)d and ns orbitals are very close, so d-electrons are as easily lost as ns electrons.

Element Electronic Configuration Oxidation States
Sc (21) [Ar] 3d¹ 4s² +2, +3
Ti (22) [Ar] 3d² 4s² +2, +3, +4
V (23) [Ar] 3d³ 4s² +2, +3, +4, +5
Cr (24) [Ar] 3d⁵ 4s¹ +2, +3, +4, +5, +6
Mn (25) [Ar] 3d⁵ 4s² +1 to +7
Fe (26) [Ar] 3d⁶ 4s² +1 to +6
CRITICAL CONCEPT:
  • Mn has maximum oxidation states (+7) in 3d series
  • +2 and +3 oxidation states are most common
  • Oxidation states increase up to middle of series then decrease
  • Due to number of unpaired electrons increasing then decreasing
Memory Tip

Oxidation states pattern: Increases up to middle (Mn = +7 max), then decreases. Common: +2, +3

Catalytic Activity

Reason for catalytic activity:

  1. Show variety of oxidation states → can form intermediate products
  2. Form interstitial compounds which can absorb/react with species
Catalyst Application
ZnO + Cr₂O₃ Manufacture of methyl alcohol
Ni, Pt, Pd Hydrogenation of vegetable oil; saturation of alkenes/alkynes
MnO₂ Decomposition of H₂O₂
TiCl₄ Manufacture of polyethene (plastic)
V₂O₅ Oxidation of SO₂ to SO₃ in H₂SO₄ manufacture
Fe Synthesis of NH₃ in Haber’s process
CRITICAL CONCEPT: Transition metals act as catalysts because:
  1. They provide active sites for adsorption
  2. Can change oxidation states easily
  3. Form weak bonds with reactants
  4. Lower activation energy of reactions
Memory Tip

Common catalysts: Fe (Haber process), V₂O₅ (Contact process), Ni (Hydrogenation), TiCl₄ (Polymerization)

Colour of Transition Metal Complexes

Mechanism: Colour is due to d-d transitions of unpaired electrons in incomplete d-orbitals.

What happens when light falls on a complex:

  1. Absorbs all white light: Appears black
  2. Reflects/transmits all light: Appears white
  3. Absorbs some, reflects rest: Shows complementary color
CRITICAL CONCEPT:
  • Complex absorbs light of particular wavelength (400-800nm)
  • Reflects complementary color → color of substance
  • Example: [Ti(H₂O)₆]³⁺ absorbs yellow light → appears violet
  • Sc³⁺ (d⁰) and Zn²⁺ (d¹⁰) are colorless (no d-d transitions)
Memory Tip

Color = d-d transitions. No color if d⁰ (Sc³⁺) or d¹⁰ (Zn²⁺). Complementary colors: Yellow ↔ Violet!

Magnetic Behaviour

Paramagnetic

  • Attracted into magnetic field
  • Due to unpaired electrons
  • Examples: Mn²⁺, Fe²⁺

Ferromagnetic

  • Can be magnetized
  • Strong attraction
  • Examples: Co, Ni

Diamagnetic

  • Slightly repelled by magnetic field
  • All electrons paired
  • Examples: Zn²⁺, Sc³⁺
μ = √[n(n+2)] BM

Where: μ = magnetic moment, n = number of unpaired electrons, BM = Bohr magneton

CRITICAL CONCEPT: By measuring magnetic moment, we can determine:
  1. Number of unpaired electrons
  2. Oxidation state of transition metal
  3. Nature of transition metal compound
Memory Tip

Magnetic behavior: Unpaired electrons = Paramagnetic, All paired = Diamagnetic. Formula: μ = √[n(n+2)] BM

Alloy Formation

Reason: Transition elements have almost similar sizes, so atoms of one metal can easily take up positions in crystal lattice of another (substitutional alloys).

Alloy Composition Properties & Uses
Brass Cu (60-80%) + Zn (20-40%) Strong, soft, flexible, doesn’t corrode, used for locks, keys, pipes
Bronze Cu (90-95%) + Sn (5-10%) Strong, brilliant, long-lasting, used for medals, coins, decorative articles
Nichrome Ni (60%) + Cr (15%) + Fe (25%) Used in electric heaters and furnace filaments
Steel Fe + C + Cr/Mn/Ni Stronger than iron, more useful properties
CRITICAL CONCEPT – Properties of alloys:
  • Comparatively cheap
  • Strong and flexible (can be made hard)
  • Long life (don’t corrode easily)
  • Durable with high melting points
  • Better conductors (non-conductor alloys also possible)
Memory Tip

Common alloys: Brass (Cu-Zn), Bronze (Cu-Sn), Steel (Fe-C-alloys), Nichrome (Ni-Cr-Fe)

Binding Energy & Melting Points

Binding Energy: Transition elements are tough due to greater binding energies. s-electrons participate in bonding, and half-filled d-orbitals also participate.

CRITICAL CONCEPT – Variation in Binding Energies:
  1. Electrons increase up to group V-B (V family) and VI-B (Cr family)
  2. Pairing of electrons starts after that
  3. Unpaired electrons become zero at group II-B (Zn, Cd, Hg)
  4. Binding forces increase up to Cr, then decrease

Melting and Boiling Points: Very high due to strong binding forces between atoms.

  • Increase up to middle of series, then decrease to minimum at end
  • Correlates with strength of binding forces
Memory Tip

Binding energy & m.p./b.p.: Increase up to middle (Cr family), then decrease. Strongest bonds in middle of series!

Formation of Complexes

Definition: Compounds containing complex molecules/ions capable of independent existence.

Component Description Example
Central Metal Atom/Ion Metal atom/ion surrounded by ligands Fe²⁺ in K₄[Fe(CN)₆]
Ligand Ion/atom/molecule donating electron pairs OH⁻, CN⁻, NH₃, H₂O
Coordination Number Number of lone pairs provided by ligands 4 in [Cu(NH₃)₄]²⁺
Coordination Sphere Central metal + ligands [Ni(CO)₄]⁰
CRITICAL CONCEPT – Chelates:

Complex compound where one or more rings form due to donation of electrons by polydentate ligand.

Example: [Pt(C₂O₄)₂]²⁻ (Dioxalato-platinate(II) ion)

Memory Tip

Complex = Central metal + Ligands. Chelates = Ring structures with polydentate ligands (like EDTA)

Nomenclature of Complexes

IUPAC Rules for Naming:

  1. Cations named before anions
  2. In coordination sphere: ligands named alphabetically followed by central metal ion
  3. Use prefixes: di, tri, tetra, penta, hexa
  4. Anionic ligands end with “o” (hydroxo, carbonato)
  5. Neutral ligands usually unchanged (ammine, aqua)
  6. Suffix ‘ate’ for negative coordination sphere
  7. Oxidation state in Roman numerals
Complex Name
K₄[Fe(CN)₆] Potassium hexacyanoferrate(II)
[PtCl(NO₂)(NH₃)₄]SO₄ Tetraammine chloro nitro-platinum(IV) sulphate
Na₃[CoF₆] Sodium hexafluoro cobaltate(III)
K₂[Cu(CN)₄] Potassium tetracyano cuprate(II)
CRITICAL CONCEPT – Writing Formulae:
  1. Symbol of central metal first
  2. Anionic ligands first, then neutral ligands
  3. Multiple ligands in alphabetical order
  4. Whole complex ion in square brackets
Memory Tip

Naming order: Ligands (alphabetical) + Metal + Oxidation state (Roman). Negative complex = Metal name ends with ‘ate’

Geometry of Complex Compounds

Tetrahedral (sp³)

  • Example: [MnCl₄]²⁻
  • 4 ligands at 109.5°
  • Coordination number: 4

Square Planar (dsp²)

  • Example: [Cu(NH₃)₄]²⁺
  • 4 ligands at 90° in plane
  • Common for d⁸ complexes

Octahedral (sp³d²)

  • Example: [Co(NH₃)₆]³⁺
  • 6 ligands at 90°
  • Most common for coordination number 6
CRITICAL CONCEPT: Geometry depends on:
  1. Coordination number
  2. Nature of central metal ion
  3. Type of ligands
  4. Hybridization of central metal
Memory Tip

Geometry: CN=4 → Tetrahedral/Square planar, CN=6 → Octahedral, CN=5 → Trigonal bipyramidal