The reaction between [Fe(CO)₂(η⁵-C₅H₄R′)X] and RNC in benzene to give [Fe(CO)_{2 –n}(η⁵-C₅H₄R′)(CNR)_nX](n= 1 or 2; R′= H, R = Bu^t, PhCH₂, or 2,6-Me₂C₆H₃, X = I or Br; R′= Me, R = Bu^t, X = I) is catalysed by [{Fe(CO)₂(η⁵-C₅H₄R′)}₂]. Mechanistic studies (42 °C) reveal that no transfer of ligands takes place between catalyst and substrate and that the reaction is light dependent. These results have been interpreted in terms of an electron-transfer process between the catalyst and the substrate. At higher temperatures (80 °C), a competing radical-chain process is also occurring, as detected by crossover experiments between catalyst and substrate. Predictions that the catalyst [{Fe(CO)₂(η⁵-C₅H₅)}₂] should activate metal carbonyl substrates in general and that ligand substitution of ligands other than CO should occur have been verified. Thus, (a) the high-yield synthesis of [Fe(η⁵-C₅H₅)(CNR)₃]I from [Fe(η⁵-C₅H₅)(CNR)₂I] and RNC (R = Bu^t or 2,6-Me₂C₆H₃), (b) the reaction between [Mo(CO)₃(η⁵-C₅H₅)I] and Bu^tNC to give [Mo(CO)₂(η⁵-C₅H₅)(CNBu^t)I] in quantitative yield (benzene, 42 °C), and (c) mechanistic studies on the reaction [Fe(CO)₄(CNBu^t)]+ Bu^tNC →[Fe(CO)₃(CNBu^t)₂]+ CO, all in the presence of [{Fe(CO)₂(η⁵-C₅H₅)}₂], are consistent with the mechanistic scheme proposed.