Mononuclear, neutral copper(I) carbenes [Bu(t)(2)P(NSiMe(3))(2)-kappa(2)N]Cu=CPh(2) and [Bu(t)(2)P(NSiMe(3))(2)-kappa(2)N]Cu=C(p-NO(2)C(6)H(4))(2) (4a,b) as well as the binuclear mu-carbene copper complex {[Bu(t)(2)P(NSiMe(3))(2)-kappa(2)N]Cu}(2)(mu-CPh(2)) (5) have been synthesized. The solid-state structures of 4b and 5 are reported. Carbene 4b is active in cyclopropanation of styrene. Reaction of 4b with (p-MeC(6)H(4))C=N(2) afforded [Bu(t)(2)P(NSiMe(3))(2)-kappa(2)N]Cu=C(p-MeC(6)H(4))(2), the corresponding mixed azine Ar(2)C=N-N=CAr'(2) and the carbene dimer Ar(2)C=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 C(2)H(4) 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.