Reactions of Isocyanides with Metal Carbyne Complexes: Isolation and Characterization of Metallacyclopropenimine Intermediates

η²-Iminoketenyl species have often been postulated as the intermediates in nucleophile-induced carbyne–isocyanide C–C coupling processes. However, such species are elusive. Here we report direct formation of η²-imino­ketenyl complexes from reactions of metalla­pentalyne with isocyanides. Our studies show that steric effects of N-substituents of the isocyanides play an important role in the stability of the three-membered metalla­cycles of the η²-imino­ketenyl complexes. Sterically bulky isocyanides, such as tert-butyl or 1-adamantyl isocyanides, inhibit bending at the isocyanide nitrogen atoms, a requirement for formation of η²-imino­ketenyl structures. Reactions of metalla­pentalyne with excess isocyanide allow the metal-bridged metalla­indene derivativesto be isolated as a result of the isocyanide insertion into the M–C_α σ bond of metalla­pentalyne.