It is generally accepted nowadays that catalysis occurring at the surface of a metal compound, such as an oxide or a sulfide, can be described in terms of the properties of the. metal ions, surrounded by ligands, on the surface of the catalyst. It is one of the attractions and also the challenge of catalytic research to find what structure on the surface causes the metal ion to be active. With the commonly used heterogeneous catalysts, the metal ion is found on the surface in conditions which vary in valency as well as in ligand surroundings. It is therefore difficult to determine what configuration is actually responsible for the catalytic activity.