General Physical and Chemical Properties (Ti to Cu)
1. Definition of Transition Element
A transition element is a d-block element that forms at least one stable ion with an incomplete d orbital. For example, Fe²⁺ (3d⁶) is a transition metal ion.
2. Shapes of 3d Orbitals
- 3dxy : Lies in the xy-plane, lobes between axes.
- 3dz² : Dumbbell with a donut shape around the z-axis.
3. Key Properties of Transition Elements
- (a) Variable Oxidation States: Due to similar energy of 3d and 4s orbitals.
- (b) Catalytic Behavior: Can change oxidation state; have vacant d orbitals.
- (c) Formation of Complex Ions: Central metal ion bonds with ligands.
- (d) Formation of Coloured Compounds: d-d transitions absorb visible light.
4. Explanation for Variable Oxidation States
- The 3d and 4s sub-shells have similar energy levels, allowing loss of different numbers of electrons.
5. Explanation for Catalytic Behavior
- Accessible oxidation states allow electron transfer.
- Vacant d orbitals allow formation of intermediates via dative bonds.
6. Explanation for Complex Ion Formation
- Transition metals have vacant d orbitals that can accept electron pairs from ligands.
Characteristic Chemical Properties
1. Complex Formation Reactions
- Examples:
- [Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺
- Ligand exchanges with Cl⁻ or OH⁻.
2. Definition of Ligand
A ligand is a species with a lone pair of electrons, forming dative covalent bonds with a metal ion.
3. Types of Ligands
- Monodentate: H₂O, NH₃, Cl⁻, CN⁻
- Bidentate: en (ethylenediamine), C₂O₄²⁻
- Polydentate: EDTA⁴⁻
4. Definition of Complex Ion
A complex is a central metal ion bonded to one or more ligands via dative bonds.
5. Geometry of Complexes
- Linear: 180° (e.g., [Ag(NH₃)₂]⁺)
- Square Planar: 90° (e.g., [Pt(NH₃)₂Cl₂])
- Tetrahedral: ~109.5° (e.g., [CuCl₄]²⁻)
- Octahedral: 90° (e.g., [Fe(H₂O)₆]³⁺)
6. Coordination Number
- Definition: Number of coordinate bonds to the central ion.
- Use ligand charges, geometry, and metal oxidation state to predict complex formulas.
7. Ligand Exchange
- E.g., [Cu(H₂O)₆]²⁺ + NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺
- Colour and geometry can change.
8. Predicting Redox Reactions with E° Values
- Use standard electrode potentials (E°) to determine feasible redox reactions.
9. Redox Reaction Examples
- MnO₄⁻ / C₂O₄²⁻ in acid
- MnO₄⁻ / Fe²⁺ in acid
- Cu²⁺ / I⁻: Cu²⁺ + 2I⁻ → CuI (white ppt) + ½I₂ (brown solution)
10. Redox Calculations
- Use mole ratios, E° values, and concentrations to calculate cell potentials and predict spontaneity.
Colour of Complexes
1. Degenerate and Non-degenerate d Orbitals
- Degenerate: Equal energy.
- Non-degenerate: Energy levels split in a ligand field.
2. Splitting of d Orbitals
- Octahedral: 3 lower (t₂g), 2 higher (e<sub>g</sub>) orbitals.
- Tetrahedral: Opposite (2 lower, 3 higher).
3. Origin of Colour
- Electron absorbs light and is promoted between split d orbitals.
- ΔE = hf, colour seen is complementary to absorbed.
4. Ligand Effects on Colour
- Strong-field ligands (e.g., CN⁻) → Larger ΔE → Absorb higher energy light.
- Weak-field ligands (e.g., H₂O) → Smaller ΔE → Absorb lower energy.
5. Examples of Colour Change
- [Cu(H₂O)₆]²⁺ (blue) → [Cu(NH₃)₄(H₂O)₂]²⁺ (deep blue)
- [Co(H₂O)₆]²⁺ (pink) → [CoCl₄]²⁻ (blue)
Stereoisomerism in Complexes
1. Types of Isomerism
- Geometrical (cis/trans):
- Square planar: [Pt(NH₃)₂Cl₂]
- Octahedral: [Co(NH₃)₄(H₂O)₂]²⁺
- Optical Isomerism:
- Chiral complexes with bidentate ligands.
- [Ni(en)₃]²⁺ has non-superimposable mirror images.
2. Polarity of Isomers
- Geometry affects overall dipole moment (polarity) and solubility.
Stability Constants (Kstab)
1. Definition
Kstab is the equilibrium constant for the formation of a complex ion.
2. Expression
- E.g., for [Cu(NH₃)₄]²⁺: Kstab= [Cu(NH₃)₄2+] / ([Cu2+][NH₃]4)
3. Calculations
- Use given values to determine concentrations at equilibrium.
4. Ligand Exchange & Kstab
- A ligand with a higher Kstab replaces one with a lower Kstab.
- Chelate effect: Multidentate ligands often form more stable complexes.
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