Co-ordination Numbers and Stereochemistry

Abstract

So far we have considered what types of species combine together to form complexes, and how stable the resultant complex may be. We must now consider the various known co-ordination numbers and how (that is in what shape) the ligands are arranged spatially around the central ion. Werner’s prediction that certain metals have preferred co-ordination numbers, for example cobalt six and platinum four, and that the ligands surround the metal in a preferred definite shape enabled great advances to be made in the understanding of co-ordination compounds. Today a wide range of physical methods are available for determining the structure of complexes. Of these methods, the most direct is X-ray crystallography by which the positions of the various atoms are located and bond lengths and angles estimated. Other useful but less direct methods include electronic and vibrational spectroscopy, dipole-moment measurements, and magnetic-susceptibility studies. As a result of such measurements on a large number of compounds, the most commonly occurring stereochemistries for the various co-ordination numbers have been established. We shall deal with these in turn but first it will prove useful to consider, with the help of some very simple theory, the possible geometries that may occur.