for characterizing compounds and reaction schemes. However, it must be realized that collectively oxidized ligands, formation of cluster-complexes and catenation may prevent the oxidation state from being well-defined. The writer would like to express his gratitude to many, but first of all to DR. CLAUS SCHÄFFER, University of Copenhagen, who is the most efficient group-theoretical engineer known to the writer; his comments and discussions have been highly valuable. The writer's colleague, Pro­ fessor FAUSTO CALDERAZZO (now going to the University of Pisa) has been most helpful in metallo-organic questions. Thanks are also due to Professors E. RANcKE- MADsEN and K. A. JENSEN for correspondence and conversations about formal oxidation numbers.