3.11. BONDING AND STRUCTURE OF METAL COMPLEXES
This section discusses some of the fundamentals helpful in understanding complexation
in water. A complex consists of a central metal atom to which neutral or
negatively charged ligands possessing electron-donor properties are bonded. The
resulting complex may be neutral or may have a positive or negative charge. The
ligands are said to be contained within the coordination sphere of the central metal
atom. Depending upon the type of bonding involved, the ligands within the
coordination sphere are held in a definite structural pattern. However, in solution,
ligands of many complexes exchange rapidly between solution and the coordination
sphere of the central metal ion.
The coordination number of a metal atom, or ion, is the number of ligand
electron-donor groups that are bonded to it. The most common coordination numbers
are 2, 4, and 6. Polynuclear complexes contain two or more metal atoms joined
together through bridging ligands, frequently OH, as shown for iron(III) in Reaction 3.43