Complex formation by transition elements

Transition elements form a large number of stable complexes because:

1. Small size and high charge density: the ions are small and carry a high positive charge, so they strongly attract ligands (lone-pair donors).
2. Vacant d orbitals of suitable energy: empty (n−1)d, ns and np orbitals are available to accept lone pairs from ligands and form coordinate (dative) bonds.
3. Variable oxidation states allow different charges and so a variety of complexes.
4. Ligand field stabilisation: the d-electrons gain extra stabilisation energy in the field of ligands.

Examples: [Fe(CN)₆]⁴⁻, [Cu(NH₃)₄]²⁺, [Ni(CO)₄], [Co(NH₃)₆]³⁺, [Ag(NH₃)₂]⁺. In contrast, s-block metals have large ions with low charge density and no d orbitals, so they form few complexes.