Formation of Complexes

Transition elements show a marked tendency to form coordination compounds (complexes) with any species -- termed a ligand -- that possesses the ability to donate a lone pair of electrons, forming a coordinate covalent (dative) bond to the metal centre. Two structural features of transition-metal ions make them especially well suited to this kind of chemistry: their ions are comparatively small and highly charged (both features that favour strong electrostatic and covalent interaction with an incoming electron-pair donor), and, crucially, they possess vacant, energetically accessible, low-energy orbitals that are able to accept the electron pair being donated by the ligand.

Because of this combination of properties -- small, highly charged metal centres with genuinely available acceptor orbitals -- transition metals form an unusually large and structurally diverse number of coordination complexes, far exceeding the complex-forming tendency of most s-block or p-block metal ions under comparable conditions. Two representative textbook examples are the hexacyanoferrate(II) complex ion, [Fe(CN)₆]⁴⁻ (iron in the +2 oxidation state, octahedrally surrounded by six cyanide ligands each donating a lone pair through carbon), and the hexaamminecobalt(III) complex ion, [Co(NH₃)₆]³⁺ (cobalt in the +3 oxidation state, octahedrally surrounded by six neutral ammonia ligands each donating a lone pair through nitrogen). …