Bonding in Coordination Compounds — Notes

The metal ion provides empty orbitals (number = coordination number) that hybridise; each ligand donates a lone pair into one hybrid orbital. Strong-field ligands (CN⁻, CO; NH₃ with Co³⁺) can pair d electrons; weak-field ligands (F⁻, Cl⁻, H₂O) cannot.

Magnetism: μ = √(n(n+2)) BM. Limitations: no explanation of colour or ligand strength; cannot predict tetrahedral vs square planar; not quantitative.

Ligands are point charges/dipoles; bonding purely electrostatic. Octahedral: $e_g$ ($d_{x^2-y^2}$, $d_{z^2}$) raised by $0.6\Delta_o$; $t_{2g}$ ($d_{xy}$, $d_{yz}$, $d_{xz}$) lowered by $0.4\Delta_o$. Tetrahedral: $e$ below $t_2$; $\Delta_t \approx \tfrac{4}{9}\Delta_o$ → always high spin.

Spectrochemical series: I⁻ < Br⁻ < SCN⁻ < Cl⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < NH₃ < en < CN⁻ < CO. $\Delta_o < P$ → high spin; $\Delta_o > P$ → low spin (d⁴–d⁷).

Colour: d–d transition absorbs light of energy $\Delta_o$; complementary colour seen; $\Delta_o = N_A hc/\lambda$. d⁰ and d¹⁰ ions colourless. Limitation: cannot explain why neutral CO is the strongest ligand (covalent/π bonding ignored).

Stepwise: $\ce{M + L <=> ML}$ ($K_1$), $\ce{ML + L <=> ML2}$ ($K_2$), … Overall $\beta_n = K_1K_2\cdots K_n$; larger β → more stable; instability constant $= 1/\beta$.

Stability increases with higher charge/smaller size of metal ion, chelation (log β: [Ni(en)₃]²⁺ 18.3 vs [Ni(NH₃)₆]²⁺ 8.6), basicity and π-acceptor strength of ligand (cyanide complexes very stable). A ligand forming a more stable complex displaces one forming a less stable complex: [Ag(NH₃)₂]⁺ (log β 7.2) → [Ag(S₂O₃)₂]³⁻ (13.5) → [Ag(CN)₂]⁻ (21.1).

Qualitative analysis: Ni²⁺ + dimethylglyoxime → red [Ni(dmg)₂]; Cu²⁺ + excess NH₃ → deep blue [Cu(NH₃)₄]²⁺; Fe³⁺ + SCN⁻ → blood-red [Fe(SCN)(H₂O)₅]²⁺; Fe³⁺ + K₄[Fe(CN)₆] → Prussian blue; EDTA titration for hardness of water.

Metallurgy: Ag and Au leached as [Ag(CN)₂]⁻, [Au(CN)₂]⁻ and displaced by Zn; Mond process: Ni + 4CO → Ni(CO)₄ (330–350 K), decomposed at 450–470 K.

Biology and medicine: haemoglobin (Fe, O₂ transport), chlorophyll (Mg, photosynthesis), vitamin B12 (Co); cisplatin (anticancer), EDTA for lead poisoning; hypo dissolves AgBr as [Ag(S₂O₃)₂]³⁻ in photography.