Stepwise formation of σ-alkynyl, vinylidene, and vinylphosphonium complexes of manganese(I)

Reaction of methyl propiolate with the diphosphinomethanide complex [Mn(CO)4{(PPh2)2CH}] (1) produces simultaneously the insertion of the alkyne into the C−H bond, giving [Mn(CO)4{(PPh2)2C−C(H)C(CO2Me)H}] (2), and the deprotonation of the alkyne mediated by the diphosphinomethanide ligand, affording the σ-alkynyl derivative fac-[Mn(C⋮C−CO2Me)(CO)3(dppm)] (3). Protonation of 3 with HBF4 at 200 K affords the corresponding vinylidene compound fac-[Mn(CC(H)CO2Me)(CO)3(dppm)]BF4 (5), which, on raising the temperature to 243 K, undergoes the spontaneous insertion of the vinylidene ligand into a Mn−P bond.