The first trinuclear clusters containing NHC ligands are described; the compound [Ru3(Me2Im)(CO)11](Me2Im=1,3-dimethylimidazol-2-ylidene) is easily converted into [Ru3(mu-H)2(mu3-MeImCH)(CO)9] by a process involving the activation of two C-H bonds of a methyl group that is an example of degradation of a metal-coordinated NHC ligand under mild conditions.

The room-temperature reactions of [Ru 3 (CO) 12 ] and [Os 3 (CO) 12 ] with a variety of N-heterocyclic carbenes (NHCs) have been studied. [Ru 3 (CO) 12 ] reacts easily with N,N′-dimethylimidazol-2-ylidene (Me 2 Im), more slowly with N-methyloxazol-2-ylidene (MeOx), and very slowly with N,N′-dimesitylimidazol-2-ylidene (Mes 2 Im) to give the corresponding CO substitution products [Ru 3 (NHC)(CO) 11 ], but it does not react with the very bulky N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (Dipph 2 Im). DFT calculations have shown that [Ru 3 (Dipph 2 Im)(CO) 11 ] is a minimum in the corresponding potential energy surface; therefore, the absence of reaction between [Ru 3 (CO) 12 ] and Dipph 2 Im has a kinetic origin associated with the large volume of this NHC. [Os 3 (CO) 12 ] reacts with Me 2 Im to give [Os 3 (Me 2 Im)(CO) 11 ]. However, MeOx is not basic enough and Mes 2 Im and Dipph 2 Im are too bulky to react with [Os 3 (CO) 12 ], which is less reactive than [Ru 3 (CO) 12 ]. Therefore, the reactions of [Ru 3 (CO) 12 ] and [Os 3 (CO) 12 ] with NHCs are strongly influenced by the electronic properties and steric demands of the NHCs and also by the intrinsic reactivity of the metal-carbonyls.

Multiple choices: The carbyne C atom of 1 and the carbide of 2 arise from an N‐methyl group of 1,3‐dimethylimidazol‐2‐ylidene. The synthesis of complex 1, which involves activation of the three CH bonds of an N‐methyl group, and that of complex 2, which requires additional activation of the CN bond, provide new examples of metal‐mediated degradation of N‐heterocyclic carbenes.

The thermolysis of the NHC triosmium cluster [Os3(Me2Im)(CO)11] (1a; Me2Im = 1,3-dimethylimidazol-2-ylidene) in toluene at reflux temperature sequentially affords the edge-bridged cluster [Os3(micro-H)(micro-kappa2-MeImCH2)(CO)10] () and the face-capped derivative [Os3(micro-H)2(micro3-kappa2-MeImCH)(CO)9] (3a). These products result from the sequential oxidative addition of one (2a) and two (3a) N-methyl C-H bonds of the original NHC ligand. The related face-capped triruthenium cluster [Ru3(micro-H)2(micro3-kappa2-MeImCH)(CO)9] (3b) has been prepared by heating the NHC triruthenium cluster [Ru3(Me2Im)(CO)11] (1b) in THF at reflux temperature. In this case, the pentanuclear derivatives [Ru5(Me2Im)(micro4-kappa2-CO)(CO)14] (4b) and [Ru5(Me2Im)2(micro4-kappa2-CO)(CO)13] (5b) are minor reaction products, but a ruthenium cluster analogous to has not been obtained. The face-capped oxazole-derived NHC triruthenium cluster [Ru3(micro-H)2(micro3-kappa2-OxCH)(CO)9] (3c; MeOx = N-methyloxazol-2-ylidene) is the only isolated product of the thermolysis of [Ru3(MeOx)(CO)11] (1c) in THF at reflux temperature.