When aryl groups are substituents in the 1,4-positions of the diazabutadiene (DAB) ligand,2 substitution of carbon monoxide by PMes in Fe(CO)s(DAB) takes place solely by a second-order process. The rate law is first order in both Fe(CO)s(DAB) and PMes. Activation parameters for the 4-fluorophenyl derivative in toluene support the associative nature of this reaction: △H*=13.6±1.0kcal/mol; △S*=-34.8±3.2 eu. Carbon monoxide replacement rates depend on the nature of the nucleophile, and increase in the series PPh3<P(OMe)3<P(n-Bu)3<PMe3. This rate also increases when the π-acceptor ability of the DAB ligand increases. When bulky tert-butyl groups are the substituents in the 1,4-positions of the DAB ligand, steric interactions become important in the six-coordinate transition state. Nucleophilic attack on this complex results in loss of the DAB ligand to give Fe(CO)3(PMe3)2. This reaction obeys a two-term rate law involving both associative (△H*=20.4±0.8kcal/mol and △S*=-18.2±2.0 eu) and dissociative (△H*=25.0±1.0kcal/mol and △S*=-8.0±3,0 eu) pathways. Factors which facilitate nucleophilic attack on iron in these coordinately saturated metallacycles and in the related Fe(CO)3(N4Me2) complex are discussed.