A key suspect arrested: The initial site of attack of a free N-heterocyclic carbene (NHC) on the triosmium carbonyl cluster [Os3(CO) 12] was found to be a carbonyl ligand. The stable zwitterionic species thus formed (blue background in the scheme) was characterized crystallographically. CO substitution proceeds from this intermediate by the loss of a CO group and migratory deinsertion. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.