AB-INITIO MOLECULAR-ORBITAL STUDY OF TRIRUTHENIUM COMPLEXES - GEOMETRICAL AND ELECTRONIC-STRUCTURE OF RU(3)CP-ASTERISK-3(MU-H)(3)(MU(3)-H)(2), RU(3)CP-ASTERISK(3)(MU-H)(6)(+), AND RU(3)CP-ASTERISK(3)(MU-H)(3) AND REARRANGEMENT OF RU(3)CP-ASTERISK(3)(MU-H)(3)(MU(3)ETA(2)-HCCR')

A theoretical analysis is presented on the structure and bonding nature of several recently synthesized cyclopentadienyl triruthenium clusters. Using ab initio RHF and simulated MP2 geometry optimizations and MP2 energetics, we could calculate structures in good agreement with the experiments for different polyhydrides and alkyne clusters. Using fragment energetics, we also give estimates of the cohesion energy and Ru-H and Ru-Ru binding energies in tri-, penta-, and hexahydrides and of the interaction energy of alkynes with the trihydride Ru(3)Cp(3)H(3). In these trimetallic complexes, the Ru-3 framework is mainly stabilized by three-center two-electron Ru-H-Ru bonds. For the alkyne clusters, the perpendicular conformation has been shown to be more stable than the more common parallel one. Of the perpendicular conformations of the Cp complex, the isomer with the larger substituent ''outside'' the metallic triangle is more stable than the isomer with the larger substituent ''inside''. For the Cp* complex, however, an analysis of the steric effects using molecular mechanics shows that the steric effects reverse the trend, making the ''inside'' isomer more stable, in agreement with the experiment. Finally, the alkyne rearrangement on the top of the metal triangle involving the exchange of the coordinated M-M bond and/or the exchange of the substituents an alkyne has been suggested to occur by the conversion of the stable perpendicular conformation into a parallel conformation intermediate via a shift or rotation of the alkyne and subsequent reversal from the intermediate.