| Abstract
| - Density functional theory predicts singlet D3h trigonal bipyramidal, D2d distorted tetrahedral, and Cs bent T-shaped structures for Os(CO)5, Os(CO)4, and Os(CO)3, respsectively. A singly bridged rather than a triply bridged structure is predicted for Os(CO)9 in accordance with the experiment. Singly bridged and unbridged structures similar in energy are predicted for Os(CO)8.
- The structures and energetics of the experimentally known Os(CO)n (n = 3−5), Os2(CO)9, and Os2(CO)8 have been investigated using density functional theory. For Os(CO)5, the lowest-energy structure is the singlet D3h trigonal bipyramid. However, the C4v square pyramid for Os(CO)5 lies only ∼1.5 kcal/mol higher in energy, suggesting extraordinary fluxionality. For the coordinatively unsaturated Os(CO)4 and Os(CO)3, a D2d strongly distorted tetrahedral structure and a Cs bent T-shaped structure are the lowest-energy structures, respectively. For Os2(CO)9, the experimentally observed singly bridged Os2(CO)8(μ-CO) structure is the lowest-energy structure. A triply bridged Os2(CO)6(μ-CO)3 structure analogous to the known Fe2(CO)9 structure is a transition state rather than a true minimum and collapses to the singly bridged global minimum structure upon following the corresponding normal mode. An unbridged (OC)5Os → Os(CO)4 structure with a formal Os → Os dative bond analogous to known stable complexes of the type (R3P)2(OC)3Os → W(CO)5 is also found for Os2(CO)9 within 8 kcal/mol of the global minimum. The global minimum for the coordinatively unsaturated Os2(CO)8 is a singly bridged (OC)4Os(μ-CO)Os(CO)3 structure derived from the Os2(CO)9 global minimum by loss of a terminal carbonyl group. However, the unbridged structure for Os2(CO)8 observed in low-temperature matrix experiments lies only ∼1 kcal/mol above this global minimum. In all cases, the triplet structures for these osmium carbonyls have significantly higher energies than the corresponding singlet structures.
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