Coordination chemistry

Coordination compounds contain a central metal atom or ion surrounded by ligands that donate electron density. Their behaviour comes from the identity of the metal, its oxidation state, the ligand set and the three-dimensional arrangement around the centre.

Metal centre, ligands and coordination number

A ligand attaches through one or more donor atoms. Monodentate ligands bind through one donor site; chelating ligands bind through several. The coordination number counts donor atoms directly attached to the metal, not simply the number of ligand molecules.

Example: in [Co(NH₃)₆]³⁺, six N donor atoms give coordination number 6.

Geometry is a structural variable, not decoration. It changes orbital splitting and therefore colour, magnetism and reactivity.

Oxidation state is a formal bookkeeping tool

The metal oxidation state is found by balancing ligand charges against the overall complex charge. It is useful for electron counting, but it is not the same thing as a measured partial charge on the metal.

For [Fe(CN)₆]^4-, six CN^- ligands contribute -6. The complex is -4 overall, so Fe is formally +2.

Ligands remove the degeneracy of the metal d orbitals. The size of the splitting helps determine electronic transitions and spin state.

Colour and magnetism come from electronic structure

When visible-light energies match allowed electronic transitions, some wavelengths are absorbed and the transmitted or reflected light appears coloured. The observed colour is not a simple label for one metal ion: ligand, geometry and oxidation state all matter.

Unpaired electrons give paramagnetism. Whether electrons pair in lower orbitals or occupy higher ones depends on the competition between ligand-field splitting and pairing energy.

Isomers and chelation

Complexes with the same formula can differ in connectivity or spatial arrangement. Cis/trans and optical isomerism can therefore change physical or biological behaviour. Chelating ligands often form especially stable complexes because several donor sites bind the same metal and dissociation requires multiple contacts to be broken or reorganized.

Worked examples

1. Oxidation state of cobalt

Solution

In [CoCl₄]²⁻, four Cl⁻ give -4 overall. Co must be +2 to produce -2.

2. Coordination number with a bidentate ligand

Solution

Three ethylenediamine ligands each donate through two N atoms, so [M(en)₃] has coordination number 6.

3. Unpaired-electron inference

Solution

A complex attracted into a magnetic-field gradient must contain at least one unpaired electron and is therefore paramagnetic.