Transition metals (and to some extent other metals) form coordination complexes in which the metal or metal ion is associated with ligands.
Examples: [Zn(NH3)4]2+, [Cu(CN)2]–, [Ni(OH2)6]2+, [CoCl4]2–, [PtCl2(NH3)2]
The coordination number of the metal in the complex is equal to the number of atoms linked directly to the metal.
The
denticity of a ligand is the
number of donor groups from that
ligand attached to the same central atom.
Ligands such as chloride ion (Cl–), cyanide ion (CN–), water (H2O) and ammonia (NH3) in the complexes above are monodentate.
Monodentate ligands form one attachment to the central metal.
The
coordination number of a complex with
only monodentate ligands is equal to the
total number of ligands.
The coordination number of
zinc in [Zn(NH3)4]2+ is 4.
copper in [Cu(CN)2]– is 2.
nickel in [Ni(OH2)6]2+ is 6.
Cordination numbers of 6 and 4 are very common. 2 is less common.