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The hydrogenation of styrene is efficiently catalyzed by H3Os4(CO)l2(I) (1), H4Os4(CO)12 (2), [H2Os4(CO)12(I)]~(3), and [H3Os4(CO)12]" (4) in decalin solution at 140 °C and 800 Torr of H2. The homogeneity of the reaction was established by a combination of chemical tests. The reaction rate is dependent on the structure of the cluster and is first order with respect to the concentrations of styrene and hydrogen; the turnover frequency, however, increases with decreasing cluster concentration. These results, together with the observed influence of added CO and other relevant data, are interpreted in terms of cluster fragmentation to yield low concentrations of highly active species of lower nuclearity. This type of fragmentation is proposed as a general phenomenon for reactions catalyzed by simple metal carbonyl clusters.

Acknowledgment. Financial support from CONICIT (Venezuela) and the Fonds der Chemischen Industrie is gratefully acknowledged. We thank V. León and J. Espidel (IVIC) for some NMR measurements, and CEPET for a Graduate Fellowship (to A.A.).

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