HYDRIDE EXCHANGE-REACTION IN TRIMETALLIC CLUSTERS - AN AB-INITIO MOLECULAR-ORBITAL STUDY OF M(3)(CO)(9)(MU-H)(3)(MU(3)-CH) (M=OS, RU)

An ab initio molecular orbital study is presented on the mechanism of the hydride exchange reaction in the trimetallic complex M(3)(CO)(9)(m u-H)(3)(mu(3)-CH) (M = Os, Ru). Based on the calculated structures and energies of many intermediates and transition states, it is proposed that the hydride exchange reaction takes place in multiple steps via t wo energetically competitive pathways. The ''inner'' pathway consists of the following steps: (1) migration of the bridging hydride H to th e nearby equatorial terminal position to form a seven-coordinated M; ( 2) clockwise turnstile rotation of three carbonyls and H around this M; (3) insertion of H into the M-C bond; (4) formation of the H*CH tr ansition-state complex as the midpoint of the degenerate exchange proc ess; and finally the reversal of steps 3, 2, and 1. The ''outer'' path way involves the following steps: (1') migration of the bridging H to the farther-away equatorial terminal position to form a seven-coordin ated M; (2') anticlockwise turnstile rotation of three carbonyls and H ; and finally a merging with step 3 of the ''inner'' pathway. Some of the above steps may take place as a concerted single step. The calcul ated barrier heights are in good agreement with the experimental value s and support the experiment in that the reaction of the Ru complex sh ould be easier than that of the Os complex. A direct mechanism without involving seven-coordinated intermediates is unlikely.