TABLE OF CONTENTS In contrast to heterogeneous catalysis, industrial applications of homogeneous catalysis are relatively scarce, largely being restricted to the speciality and pharmaceutical sectors. Homogeneous catalysts have been well researched, since their catalytic centres can be relatively easily defined and understood, but difficulties in separation and catalyst regeneration have prevented their wider use. These challenges are the focus of much current research.
The most widely used homogeneous catalysts are simple acids and bases which catalyse well-known reactions such as ester and amide hydrolysis, and esterification. Such catalysts are inexpensive enough that they can be neutralized, easily separated from organic materials, and disposed of. This, of course, is not a good example of green chemistry and contributes to the huge quantity of aqueous salt waste generated by industry.
Many of the green benefits of homogeneous catalysis, especially that of high selectivity, arise from 'designer' catalysts made from transition metals and appropriate ligands. It is the partially filled d-orbitals that make transition metals attractive catalysts; these orbitals are of relatively high energy enabling electrons to be readily transferred in or out. Ligands bond with transition metals via these partially filled d-orbitals. The maximum number of metal-ligand bonds being determined by the 18-electron rule (Equation 4.2), where n is the number of d-electrons for the particular oxidation state of the metal and CN is the metal co-ordination number, or the number of metal-ligand ? bonds. Complexes which are co-ordinatively unsaturated, i.e.
