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carbonylation. Compound 9 contains a square pyramid of five osmium atoms with an Os(CO)3 capping group. It seems reasonable to expect that the analogous Ru6-(CO)17(m4-S) could be prepared by the decarbonylation of 3; however, this has not yet been achieved.The addition of yet another ruthenium carbonyl fragment results in the formation of Ru7(CO)19(m-CO)2(m4-S) (4). This product was made independently by the addition of a mononuclear fragment to 3. The structure of 4 shows that it contains two Ru(CO)4 groups which bridge diametrically opposed basal edges of the square pyramid and was formed by the replacement of a second bridging CO ligand in 3 with a Ru(CO)4 group. The osmium homologue of 4 has not yet been reported. The closest related osmium compound is Os7(CO)19(g4-S) which contains a sulfurbridged square pyramid of five osmium atoms that is fused to a trigonal-bipyramidal cluster of five osmium atoms through a triangular face.13 Compounds 2, 3, and 4 can be degraded by treatment with CO at 98 °C. Curiously, small amounts of 4 were formed in the degradation of 3 and small amounts of 3 and 4 were obtained in the degradation of 2. The degradation reactions will lead to the formation of mononuclear ruthenium fragments which could be added to unreacted clusters. It is believed that under suitable conditions cluster enlargement and degradation will be ongoing and competing reactions in solutions that contain the appropriate species. These transformations are summarized in Scheme I.Acknowledgment. The research was supported by the National Science Foundation under Grant No. CHE-8612862.

The mechanisms of chemical reactions cannot be determined by experiment alone because reactions take Michael J. S. Dewar was bom In Ahmednagar, India, and became an American citizen In 1980. He received his B.A., M.A., and D.Phil. from Oxford University, followed by a postdoctorate and fellowship at the same Institution. Dewar has held professorships at Queen Mary College of the University of London, the University of Chicago, and the University of Texas at Austin, and Is now on the faculty of the University of Florida. He Is a Fellow of the Royal Chemistry Society (London) and a member of the National Academy of Sciences.Caoxian Jie was born In China on August 29, 1941 and received his B.S.

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AMI calculations for the Cope rearrangements of 1,5-hexadiene (1) and its 2-phenyl (6), 3-phenyl (9), 3-methyl (15), 2,4-diphenyl (10), and 2,5-diphenyl (11) derivatives, via chair transition states, support the Doering biradicaloid mechanism previously predicted by MINDO/3. The relative rates for 1, 6, 9, and 15 are reproduced closely, the calculated heats of activation being uniformly too large by 3.35 kcal/mol. Larger deviations for 10 and 11 can be attributed to solvent effects and experimental error.The degenerate rearrangements of 1, 6, and 11 were predicted to involve 1,5-cyclohexylene biradicaloids as marginally stable intermediates, the lengths of the QQ and C3C4 bonds being 1.60-1.65 Á. The other rearrangements were predicted to be concerted but not synchronous. Calculations for 1 and 6 with the QQ and C3C4 bond lengths set equal to 2.06 Á, as expected for a synchronous transition state, predicted retardation by phenyl as predicted by PMO theory. Deuterium kinetic isotope effects calculated for 1, 6, and 11 agreed with Gajewski's measurements, within the limits of error of the calculations and experiments.

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