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)(-ASTERISK(3)(MU-H)(3) AND REARRANGEMENT OF RU(3)CP-ASTERISK(3)(MU-H)(3)(MU(3)ETA(2)-HCCR')(), AND RU(3)CP)

A theoretical analysis is presented on the structure and bonding natur e of several recently synthesized cyclopentadienyl triruthenium cluste rs. Using ab initio RHF and simulated MP2 geometry optimizations and M P2 energetics, we could calculate structures in good agreement with th e experiments for different polyhydrides and alkyne clusters. Using fr agment energetics, we also give estimates of the cohesion energy and R u-H and Ru-Ru binding energies in tri-, penta-, and hexahydrides and o f the interaction energy of alkynes with the trihydride Ru(3)Cp(3)H(3) . In these trimetallic complexes, the Ru-3 framework is mainly stabili zed by three-center two-electron Ru-H-Ru bonds. For the alkyne cluster s, the perpendicular conformation has been shown to be more stable tha n 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 subs tituent ''inside''. For the Cp complex, however, an analysis of the s teric effects using molecular mechanics shows that the steric effects reverse the trend, making the ''inside'' isomer more stable, in agreem ent with the experiment. Finally, the alkyne rearrangement on the top of the metal triangle involving the exchange of the coordinated M-M bo nd and/or the exchange of the substituents an alkyne has been suggeste d to occur by the conversion of the stable perpendicular conformation into a parallel conformation intermediate via a shift or rotation of t he alkyne and subsequent reversal from the intermediate.