eta(2)-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 eta(2)-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 eta(2)-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 eta(2)-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-alpha sigma bond of metallapentalyne.