Mononuclear, neutral copper(I) carbenes [(Bu2P)-P-t(NSiMe3)(2)-kappa N-2]Cu=CPh2 and [(Bu2P)-P-t(NSiMe3)(2)-kappa N-2]Cu=C(p-NO2C6H4)(2) (4a,b) as well as the binuclear u-carbene copper complex {[(Bu2P)-P-t(NSiMe3)(2)-kappa N-2]Cu}(2)(mu-CPh2) (5) have been synthesized. The solid-state structures of 0 and 5 are reported. Carbene 4b is active in cyclopropanation of styrene. Reaction of 4b with (p-MeC6H4)C=N-2 afforded [(Bu2P)-P-t(NSime(3))(2)-kappa N-2]Cu=C(p-MeC6H4)(2), the corresponding mixed azine Ar2C=N-N=CAr'(2) and the carbene dimer Ar2C=CAr'(2), demonstrating an important pathway for carbene loss in cyclopropanation of olefins via reaction of the copper(I) carbene with nucleophilic diazo compounds. Complex 5 reacted with C2H4 or styrene to give 1:1 mixtures of the terminal carbene 4a and of the corresponding copper(I) olefin complexes, proving unambiguously that binuclear complexes like 5 are not relevant for copper-catalyzed cyclopropanation of olefins with diazo compounds. The electronic structure of 4b was investigated by means of DFT calculations.