How metal ions bind ligands to form complex compounds
Coordination compounds contain a central metal atom or ion bonded to surrounding molecules or ions called ligands, which donate lone pairs of electrons. The number of donor atoms bonded to the metal is the coordination number; in [Co(NH₃)₆]³⁺ it is six. Ligands can be monodentate, like NH₃, or polydentate, like ethylenediamine and EDTA, which form stable ring structures called chelates. Werner’s theory distinguished primary valency (oxidation state) from secondary valency (coordination number).
IUPAC naming lists ligands alphabetically before the metal, with the oxidation state in Roman numerals. Coordination compounds show structural isomerism, such as ionisation and linkage isomerism, and stereoisomerism, including geometrical cis–trans and optical isomers. Valence bond theory explains geometry through hybridisation, such as d²sp³ for octahedral complexes. Crystal field theory explains colour and magnetism: ligands split d orbitals, and strong-field ligands like CN⁻ cause electrons to pair, giving low-spin complexes.
Why it matters
Haemoglobin, which carries oxygen in blood, and chlorophyll in plants are coordination compounds of iron and magnesium. EDTA is used to measure water hardness and treat metal poisoning, cisplatin is a cancer medicine, and complexes are used in photography, electroplating and the extraction of gold and silver.
Key terms
- Ligand
- An ion or molecule that donates a pair of electrons to a central metal atom.
- Coordination number
- The number of ligand donor atoms directly bonded to the central metal.
- Chelate
- A ring-shaped complex formed when a polydentate ligand binds a metal through several donor atoms.
- Crystal field splitting
- The separation of a metal’s d-orbital energies caused by surrounding ligands.