Complexes of the type (R3P)(OC)4OsM(CO)5 (R = Me, OMe, Ph, etc.; M = Cr, Mo, W) have been prepared from the reaction of Os(CO)4(PR3) and M(CO)5(THF) in hexane. The structures of (Me3P)(OC)4OsCr(CO)5 ( 1) and (Me3P) (OC)4OsW(CO)5 (2) have been determined by X-ray crystallography: Compound 1 crystallizes

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A carbene transfer agent, 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene-silver(I)chloride ([IMes]-AgCl), was treated with various osmium clusters. Reaction of ([IMes]AgCl) with [Os 3 (µ-H) 2 (CO) 10 ] gave [Os 3 (µ-H)(µ-Cl)(CO) 9 (IMes)] (1), an osmium N-heterocyclic carbene complex. The reaction of ([IMes]AgCl) with the cluster [Os 3 (CO) 10 (CH 3 CN) 2 ] yielded two products, [Os 3 (µ-Cl)(CO) 10 (µ-Ag(IMes)] (2) and [(IMes-H)][Os 3 (µ-Cl)(CO) 10 ](µ 4 -Ag)[Os 3 (µ-Cl)(CO) 10 ] (3). Compound (2) is a heterobimetallic carbene complex resulting from the complete incorporation of ([IMes]AgCl). Transmetalation using [(IMes)AgCl] and the cluster [Os 4 (µ-H) 4 (CO) 12 ] in the presence of trimethylamine-N-oxide resulted in the formation of [Os 4 (µ-H) 4 (CO) 11 (n • IMes)] (4), where (n denotes normal carbene bonding, i.e., through C 2 ). [Os 4 (µ-H) 4 (CO) 10 (IMes) 2 ] (5) was formed using the activated cluster [Os 4 (µ-H) 4 (CO) 10 (CH 3 CN) 2 ] and [(IMes)AgCl]. Compound (5) was found to be sensitive to silica gel column chromatography in the presence of dichloromethane decomposing to [Os 4 (µ-H) 3 (µ-Cl)(CO) 11 (IMes)] ( 6), which possessed a butterfly metal skeleton. The ruthenium analogue of ( 4), [Ru 4 (µ-H) 4 (CO) 11 (IMes)] ( 7), was prepared and identified spectroscopically but was found to be thermally unstable. Thermolysis of (4) in benzene yielded four products. The abnormal carbene complex [Os 4 (µ-H) 4 (CO) 11 (a • IMes)] (10) (a denotes abnormal carbene bonding through C 3 or C 4 ), an isomer of (4), and {Os 4 (µ-H 11), an unusual complex resulting from the activation of benzene and activation of three C-H bonds on IMes, were isolated in modest yields. Two higher-nuclearity clusters, {Os 5 (µ 5 -C)(CO) 14 [η-C-η-NC 2 H 2 (Mes)]} ( 8) and [Os 6 (µ-H) 4 (µ 5 -C)(CO) 15 (IMes)] ( 9), both of which contain carbide ligands, were also isolated. {Os 4 (µ-H)(CO) 10 [(η-C)N(Mes)C 2 H 2 NC 6 H 2 (CH 3 ) 2 (η 2 -C)(η-C)CHC(CH 3 )-C 2 H 2 (η 2 -C)]} (12), an analogue of (10), resulted from the thermolysis of (4) in toluene indicates that the reaction is potentially general and exhibits some regioselectivity. Crystal structure determinations were completed on all 11 osmium cluster complexes.