Unsaturated trinuclear osmium carbonyls: comparison with their iron analogues.

Comparison of theoretical and experimental structural parameters as well as nu(CO) frequencies for Os(3)(CO)(12) suggests that the density functional theory (DFT) method MPW1PW91 with a suitable ECP basis set including relativistic effects is a reliable method for predicting structures and vibrational frequencies of third row transition metal carbonyl derivatives. Using this method the structures of the unsaturated trinuclear osmium carbonyl derivatives Os(3)(CO)(n) (n = 11, 10, 9) have been investigated for comparison with their iron carbonyl analogues. For Os(3)(CO)(11) the global minimum has micro-CO groups bridging each edge of the Os(3) triangle in contrast to its iron analogue predicted to have two micro(3)-CO groups bridging all three iron atoms. An alternative Os(3)(CO)(11) structure having only terminal CO groups, similar to that observed experimentally by Bentsen and Wrighton (J. G. Bentsen and M. S. Wrighton, J. Am. Chem. Soc., 1987, 109, 4518) in the photolysis of Os(3)(CO)(12) in low temperature hydrocarbon matrices, is predicted to lie approximately 7 kcal mol(-1) above this global minimum. The global minimum for Os(3)(CO)(10) has one face-bridging micro(3)-CO group and one edge-bridging micro-CO group. For Os(3)(CO)(9) the global minimum has an Os(3) scalene triangle with different metal-metal distances suggesting one single, one double, and one triple metal-metal bond similar to its iron analogue.

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