Kinetic study of ligand-substitution reactions of (1–3-η-allyl)di-carbonylnitrosyliron complexes: factors affecting the stability of σ- and η-allyl complexes

Abstract

The 1–3-η-allyl complexes [(η-C₃H₄X)Fe(CO)2NO](I; X = 1-Me, 1-Ph, and 2-Me) react with phosphine ligands [L = PPh₃ or P(OEt)₃]via a second-order associative mechanism to give [(η-C₃H₄X)Fe(CO)(NO)L], (II); for X = 2-Me and L = PPh₃, a parallel first-order dissociative route is operative. For X = 1-Cl, 1-CN, 2-Cl, and 2-Br intermediate five-co-ordinate complexes [(σ-C₃H₄X)Fe(CO)₂(NO)L], (III) are formed by a second-order associative process with the ligand [L = PPh₃, PBuⁿ₃, PPh₂Et, P(OEt)₃, or P{(OCH₂)₃OEt}]. Species (III) then rearrange to [(σ,η-(C₃H₄X)Fe(CO)(NO)L], (IV), via a first-order associative intramolecular chelation, with loss of CO. The above process with P(OEt)₃ shows a parallel second-order path of ligand disubstitution via CO insertion between the metal atom and the σ-allyl group, to give but-3-enoyl complexes, (V). Activation parameters for these processes have been estimated; the influence of the X substituents and of the entering L ligands has been examined.