Reaction of [Fe(CO)4(η2-BTMSA)] (1) (BTMSA = Me3SiCCSiMe3) with trimethylphosphine proceeds by stepwise loss of CO ligands yielding [Fe(CO)3(PMe3)(η2-BTMSA)] (2) and [Fe(CO)2(PMe3)2(η2-BTMSA)] (3), respectively. This demonstrates the CO dissociative nature of substitution in 1 toward PMe3, as established in analogous M(CO)4(η2-alkyne) (M = Fe, Ru, Os) compounds. The structures of all three compounds have been determined by X-ray crystallography to assess the effect of CO substitution by PMe3 on the metalalkyne bonding in these molecules.Key words: iron, carbonyl, alkyne, substitution, phosphine, steric effects.