Treatment of the methyl complex Tp′(CO)(PhC 2 Me)W-Me (1) or Tp(CO)(PhC 2 Ph)W-Me (2) with trityl cation yields the corresponding methylene complex [Tp′(CO)(PhC 2 Me)WdCH 2 ]-[PF 6 ] (3) (Tp′ ) hydridotris-(3,5-dimethylpyrazolyl)borate) or [Tp(CO)(PhC 2 Ph)WdCH 2 ][PF 6 ] (4) (Tp ) hydridotris(pyrazolyl)borate). The carbene complex (3) is unusually persistent for a methylene complex. Complexes 3 and 4 display electrophilic behavior, as evidenced both by addition of nucleophiles and by methylene transfer to electron rich olefins. When bound to either carbene 3 or 4, arylimines are activated toward nucleophilic attack by ethyl diazoacetate (EDA). The net result of the addition of excess arylimine followed by excess EDA to a solution of either 3 or 4 is catalytic aziridine formation. X-ray diffraction studies have revealed the structures of [Tp′(CO)(PhC 2 Me)W-CH 2 PMe 3 ][PF 6 ] (7) and [Tp′(CO)(PhC 2 -Me)W-CH 2 N(Me)dC(H)(Ph)][PF 6 ] (15). The relative stabilities of 3 and 4 are discussed as well as the mechanism of aziridine catalysis.