Reaction mechanisms of metal–metal-bonded carbonyls. Part 21. Reactions of bromine with some substituted binuclear carbonyls

Abstract

The kinetics have been followed of reactions of bromine in cyclohexane or decalin with a number of axially substituted complexes [M₂(CO)_{10 –n}L_n](M₂= Mn₂, MnRe, or Re₂; n= 1 or 2; L = phosphorus-donor ligand). The reactions are fast and the major products are the complexes cis-[M(CO)_{5 –m}BrL_m](m= 0 or 1). Orders as high as three in [Br₂] are observed and there is kinetic evidence for very rapid pre-association of Br₂ with [Mn₂(CO)₈(PEt₂Ph)₂] before the relatively much slower oxidation step. Reaction of [Mn₂(CO)₈{P(OMe)₃}₂] in other solvents shows similar evidence. The rates of oxidation of [M₂(CO)₈(PPh₃)₂](M₂= Mn₂, MnRe, or Re₂) show a close inverse relationship to their ease of polarographic reduction. Rates of oxidation of the dimanganese complex generally increase with the basicity of the substituents although the bis(tricyclohexylphosphine) and, probably, the bis(triphenylphosphine) complexes show evidence of steric retardation. The data are all consistent with initial electrophilic attachment of one or more Br₂ molecules at O atoms of the carbonyl ligands. The subsequent steps are not defined by the kinetics. The effects of using MeOH, CH₂Cl₂, and CCl₄ as solvents were also examined. The rate equations changed and their form shows evidence for formation of [Mn₂(CO)₈{P(OMe)₃}₂(Br₂)] and [Mn₂(CO)₈{P(OMe)₃}₂(Br₂)₂] adducts in MeOH, and of only the former in CH₂Cl₂ and CCl₄. Overall rates are much increased but no simple rationalization is possible.