The electronic structures of the black Roussinate anion, the string bag cluster model of the active center of nitrogenase, and its cluster fragments, the red Roussinate dianion, were calculated using the close shell CNDO/2 (S, D scheme) method. From the analyses of the bonding properties on the basis of Muelliken populations, charge densities, MO energies, and MO characters, the following results were revealed. An intensive electron delocalization was found in these 2 clusters. In the process of electronic transfer from the red Roussinate to the black Roussinate, the bridging S atom (Sb) was the electron donor. There existed a certain degree of M-M (where M is metal) bonding in these 2 clusters with the strength of the M-M bond near to that of M-Sb bond; the chief contribution to these bonds was the interaction between the s, p, and dz2 orbitals on the metal atoms and the s and pz orbitals on the S atoms. The contribution of the p interaction between the d orbitals on the metal atoms to the bonding of the cluster skeleton is quite small.

Key words: IRON COMPLEX, NITROGENASE, MOLYBDENUM COMPLEX, CNDO APPROXIMATION, CLUSTER COMPOUND, ENERGY LEVELS, ELECTRONIC STRUCTURE, MOLECULAR ORBITAL THEORY, BONDING, SULFIDE COMPLEX