Key Points

Alfred Werner proposed that metals have two types of valencies: a primary valency (ionisable, corresponds to oxidation state) and a secondary valency (non-ionisable, corresponds to coordination number).

A coordination entity consists of a central metal atom/ion bonded to a fixed number of molecules or ions called ligands. The central atom acts as a Lewis acid, and ligands act as Lewis bases.

The coordination number is the number of ligand donor atoms bonded to the central metal. The central atom and its attached ligands are enclosed in a square bracket, known as the coordination sphere.

Ligands are classified by their denticity: unidentate (one donor atom, e.g., $\text{Cl}^-$, $\text{NH}_3$), didentate (two donor atoms, e.g., ethane-1,2-diamine or en), and polydentate (several donor atoms, e.g., EDTA).

A didentate or polydentate ligand that binds a single metal ion to form a ring structure is a chelate ligand, which increases stability. An ambidentate ligand can coordinate through two different atoms, like $\text{NO}_2^-$ (via N or O).

Homoleptic complexes have a central metal bonded to only one type of ligand, e.g., $[\text{Co}(\text{NH}_3)_6]^{3+}$. Heteroleptic complexes have more than one type of ligand, e.g., $[\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+$.

Name the cation first, then the anion. Ligands are named alphabetically before the metal. Anionic ligands end in -o. If the complex is an anion, the metal's name ends in -ate. The metal's oxidation state is in Roman numerals.

Structural isomers have the same formula but different atom-to-atom bonds. Types include ionisation (exchange of ligand with counter-ion), linkage (ambidentate ligand bonding), coordination (ligand exchange between complex ions), and solvate isomerism.

Geometrical isomers have the same bonds but different spatial arrangements. Common types are cis (adjacent) and trans (opposite) in square planar ($[\text{Ma}_2\text{b}_2]$) and octahedral ($[\text{Ma}_4\text{b}_2]$) complexes, and fac-mer in octahedral ($[\text{Ma}_3\text{b}_3]$) complexes.

Optical isomers (enantiomers) are non-superimposable mirror images of each other and are optically active (chiral). This is common in octahedral complexes with didentate ligands, such as $[\text{Co}(\text{en})_3]^{3+}$.

VBT explains bonding in terms of orbital hybridisation. The geometry is determined by the type of hybridisation: $sp^3$ (tetrahedral), $dsp^2$ (square planar), $d^2sp^3$ or $sp^3d^2$ (octahedral).

Inner orbital complexes use inner $(n-1)d$ orbitals for hybridisation (e.g., $d^2sp^3$), are typically low spin, and are formed with strong-field ligands. Outer orbital complexes use outer $nd$ orbitals (e.g., $sp^3d^2$), are high spin, and are formed with weak-field ligands.

CFT is an electrostatic model where ligands create a field that splits the degeneracy of the central metal's d-orbitals. This splitting explains the magnetic properties and color of coordination compounds.

In an octahedral field, the five d-orbitals split into two sets: a lower energy $t_{2g}$ set ($d_{xy}, d_{yz}, d_{xz}$) and a higher energy $e_g$ set ($d_{x^2-y^2}, d_{z^2}$). The energy difference is the crystal field splitting energy, $Δ_o$.

The spectrochemical series arranges ligands by their ability to cause d-orbital splitting: $\text{I}^- < \text{Cl}^- < \text{H}_2\text{O} < \text{NH}_3 < \text{CN}^- < \text{CO}$. Strong-field ligands cause large splitting ($Δ_o > \text{P}$), leading to low spin complexes, while weak-field ligands cause small splitting ($Δ_o < \text{P}$), leading to high spin complexes.

The color is due to d-d electron transitions. The complex absorbs light of a specific wavelength to promote an electron from the lower energy $t_{2g}$ orbital to the higher energy $e_g$ orbital. The observed color is the complementary color of the light absorbed.

Metal carbonyls exhibit synergic bonding. It involves a σ bond from the donation of electrons from carbonyl carbon to the metal, and a π back-bond from the donation of electrons from a filled metal d-orbital to the vacant antibonding π* orbital of CO.

They are vital in many areas: chlorophyll (Mg complex) and haemoglobin (Fe complex) in biology, cis-platin in cancer therapy, EDTA in estimating water hardness, and in metallurgical processes for extracting metals like gold and silver.