Homoleptic mononuclear and binuclear osmium carbonyls Os(CO)n(n = 3-5) and Os2(CO)n (n = 8, 9): comparison with the iron analogues.
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 D(3h) trigonal bipyramid. However, the C(4v) square pyramid for Os(CO)5 lies only approximately 1.5 kcal/mol higher in energy, suggesting extraordinary fluxionality. For the coordinatively unsaturated Os(CO)4 and Os(CO)3, a D(2d) 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(mu-CO) structure is the lowest-energy structure. A triply bridged Os2(CO)6(mu-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(mu-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 approximately 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.
Xu
Bing
B
Institute of Chemical Physics, Beijing Institute of Technology, Beijing, China.
Li
Qian-Shu
QS
Xie
Yaoming
Y
King
R Bruce
RB
Schaefer
Henry F
HF