Metal–ligand interactions

Abstract

The interactions of metals with other solutes – usually termed ligands – are of widespread importance, in industry, biochemistry and medicine, as well as environmental science. Introductory accounts in relation to natural waters have been given by Stumm & Morgan (1996) and Langmuir (1997), while McBride (1994) focuses on soil environments. The intention here is to lay the foundations for considering the interactions of metal cations with humic substances. Coordination Cations in aqueous solution are not independent entities. They interact with molecules or ions containing free pairs of electrons (ligands), to achieve a completed electronic outer shell, a process known as coordination. In the simplest case, the ligands are water molecules. The number of ligands in nearest-neighbour positions is called the coordination number, for which the commonest values are two, four and six. Surrounding the inner coordination sphere is an outer sphere, comprising water molecules with a more ordered structure than the bulk solvent (Fig. 5.1). Complex formation When a cation combines with a ligand other than water, the resulting entity is referred to as a complex. Outer-sphere complexes are formed when the ligand is situated in the outer coordination sphere, forming no direct bonds to the metal ion (Fig. 5.1) and held by the relatively weak forces of electrostatic attraction and hydrogen bonding.