The electronic structures of red roussinate and black roussinate

The electronic structures of the black roussinate monoanion, the first step model compound of the Fuzhou model-I (the “String bag cluster model”) of the active center of nitrogenase, and its “cluster fragments”, the red roussinate dianion, have been calculated by using the close shell CNDO/2 (S,D scheme) method. From the analysis of the bonding properties on the basis of Mulliken populations, charge densities, MO energies and MO characters, the following results have been revealed. The intensive electron delocalization has been found in these two clusters. In the process of electronic transfer from the red roussinate to the black roussinate, the sulfur is an electron donor. There exists a certain degree of M-M bonding in these two clusters. The strength of M-M bond is near to that of M-Sb bond; the main contribution to these bonds is the interaction between the s, p, and dz2 orbitals on the metal atoms, and the s and pz orbitals on the sulfur atoms. The contribution of the π interaction between the d orbitals on the metal atoms to the bonding of the cluster skeleton is quite small.