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.