28.2.4—Complex ions

A complex is a molecule or ion in which a central metal atom or ion is bonded to surrounding ligands through coordinate bonds. The bracketed species has its own overall charge.

Separate the complex ion from counter-ions outside the brackets. Count ligand donor atoms to determine coordination number, then check total charge.

In [CoCl₄]²⁻, Co is central, four chloride ligands coordinate, and the bracketed species carries −2 charge.

The counter-ions are not ligands inside the complex, and coordination number is not always equal to the number of written ligand molecules for polydentate ligands.