New µ-methylene-dicobalt complexes and evidence for µ-methylene to terminal methylene transformations

New µ-alkylidene complexes [CO2(CO)4(µ-CHR)(µ-dppm)] and [Co2(CO)4(µ-CH2)(µ-CHR)(µ-dppm)], R H or CO2Et, dppm = Ph2PCH2PPh2, have been prepared by displacement of µ-CO groups from [CO2(CO)4(µ-CO)2(µ-dppm)]on reaction with CHRN2; dynamic n.m.r. data indicate rapid µ-CH2 to terminal methylene transformations in these complexes.