Unsaturated trinuclear osmium carbonyls: comparison with their iron analogues

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Abstract
Comparison of theoretical and experimental structural parameters as well as ν(CO) frequencies for Os3(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 Os3(CO)n (n = 11, 10, 9) have been investigated for comparison with their iron carbonyl analogues. For Os3(CO)11 the global minimum has µ-CO groups bridging each edge of the Os3 triangle in contrast to its iron analogue predicted to have two µ3-CO groups bridging all three iron atoms. An alternative Os3(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 Os3(CO)12 in low temperature hydrocarbon matrices, is predicted to lie 7 kcal mol−1 above this global minimum. The global minimum for Os3(CO)10 has one face-bridging µ3-CO group and one edge-bridging µ-CO group. For Os3(CO)9 the global minimum has an Os3 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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