The custer [ru3(CO)10(µ2-NO)2] reacts with cO: to form the species [ru3(CO)12], [ru3(CO)10(µ2NO)(µ2-NCO)], [Ru4N(CO)12(µ2NO)], and [ru4N(CO)12(µ2-NCO)], evidence suggesting the formation of a reactive nitrido-or nitrene–niteosyl intermediate; X-ray analysis shows that both of the tetranuclear nitrido species adopt a 64-electron‘butterfly’ structure in which the ‘hinge’ metal-meatl vector is long and supports a(µ2-NO) or a (µ2NCO) bridging group.