Chalcogen derivatives of iron carbonyls. Part 7. Kinetics and mechanism of carbonyl replacement in a nonacarbonylthiodicobaltiron

Abstract

The kinetics of ¹⁴CO exchange and CO replacement in [FeCo₂(CO)₉S] have been studied with a variety of ligands. The rate equation is K_obs.=k₁+k₂[L], in which the ligand-independent path is often negligible, except with very poor nucleophiles such as CO or AsPh₃. The ligand-dependent path indicates the very high electrophilicity and great discriminating power among the ligands of the co-ordination site, suggesting that bond making plays an important role in the transition state. The probable mechanism is discussed in terms of steric accessibility of the co-ordination site and in terms of a bonding model for the cluster complex. The rates of the successive CO substitutions with PBu₃ indicate that the ligand already bonded to the cluster affects the reactivity of the adjacent metal atom, i.e. CO dissociation is practically unaffected, whereas ligand co-ordination is greatly decreased.