Kinetic studies of CO/L substitution reactions of the well-known organometallic complex (mu-pdt)[Fe(CO)(3)](2) (pdt = 1,3-propanedithiolate), complex 1, and its sulfur-oxygenated derivative (mu-pst)[Fe(CO)(3)](2) (pst = 3-sulfenatopropane-1-thiolate), 1-O, have been carried out with the goal of understanding the influence of the sulfenato ligand on the activation barrier to ligand substitution in such diiron carbonyl complexes which consists of two components: intramolecular structural rearrangement (or fluxionality) and nucleophilic attack by the incoming ligand. The CO/PMe3 substitution reactions of complex 1 follow associative mechanisms in both the first and the second substitutions; the second substitution is found to have a higher activation barrier for the overall reaction that yields 1-(PMe3)(2). Despite the increased electrophilicity of the Fe(CO)(3) unit in 1-O versus 1, the former reacts more sluggishly with PMe3, where practical kinetic measurements are at such high temperatures that CO dissociation parallels the associative path. Kinetic studies have established that in complex 1-O both the first and the second CO/CN-substitutions proceed via associative paths with higher E-act barriers than the analogous reactions with complex 1. Theoretical calculations (density functional theory) have been used in conjunction with variable temperature C-13 NMR spectral studies to examine the energy barriers associated with rotation of the Fe(CO)(3) unit. The activation energy required for rotation is higher in the sulfenato than in the analogous thiolato complexes. Thus, the greater barrier to structural deformation in 1-O inhibits its ability to expand its coordination number as compared to the thiolate,1, resulting in slower reaction rates of both PMe3 and CN- substitution reactions.