The range of processes of degradation−transformation of NHC ligands in the coordination sphere of a transition metal has been enlarged. The NHC-acyl ligand of the complex Os{κ 2-C,C-[C(O)CH 2 ImMe]}Cl(P i Pr 3) 2 (1) undergoes a complex rearrangement promoted by internal alkynes to give Os{κ 2-C,N-[CH 2 ImMe]}Cl(CO)(P i Pr 3) 2 (2). Mechanistic studies have revealed that the degradation involves a catalytic alkyne-mediated deinsertion of CO from the acyl moiety to afford Os{κ 2-C,C-[CH 2 ImMe]}Cl(CO)(P i Pr 3) 2 (3), followed by a thermally activated stoichiometric 1,2-methylene shift from N to C. The catalytic activity of the alkynes depends upon their substituents, decreasing in the sequence diphenylacetylene > 1-phenyl-1-propyne > 3-hexyne > 2-butyne. Phenylacetylene tautomerizes in the metal coordination sphere to afford the stable vinylidene Os{κ 2-C,C-[C(O)CH 2 ImMe]}Cl(CCHPh)(P i Pr 3) 2 (4), which experiences the coupling of the acyl moiety and the vinylidene ligand under a carbon monoxide atmosphere. The addition of HBF 4 •OEt 2 to the resulting complex Os{κ 2-C,C-[C(CHPh)C(O)CH 2 ImMe]}Cl-(CO) 2 (P i Pr 3) (5) leads to [Os{κ 2-O,C-[OC(CHCHPh)CH 2 ImMe]}Cl(CO) 2 (P i Pr 3)]BF 4 (6) containing an NHC-(α,βunsaturated ketone) ligand.