Solvent and structural effects in substitution reactions of dicarbonyliodo(η-cyclohexa-1,3-dienyl) and dicarbonyliodo(η-cyclo-hepta-1,3-dienyl)iron complexes

Article abstract of DOI:10.1016/S0020-1693(00)93853-2

A comparison of the kinetics of substitution by phosphite ligands of dicarbonyliodo(η-cyclo-hexa-1,3-dienyl) and dicarbonyliodo(η-cyclo-hepta-1,3-dienyl)iron complexes is reported. In all cases a dissociative mechanism is observed. The variation of rate with the size of the attached ring is in the sequence: Cyclo-hexadienyl > Cycloheptadienyl > Cyclopentadienyl which is not in accord with the observed carbonyl stretching frequencies. However, detailed solvent studies show that for the six and seven-membered ring compounds the transition state is more polar than the ground state which is the opposite of that observed for the cyclopentadienyl series. It is also suggested that solvents with comparable donor and acceptor qualities stabilize the dissociative transition state by a push-pull type mechanism.