Reaction of di-μ-phenylthio-bis(tricarbonyliron)(Fe-Fe) with triphenyl-phosphine: A detailed kinetic and mechanistic study

Di-μ-phenylthio-bis(tricarbonyliron)(Fe-Fe) undergoes a two-step CO substitution reaction with triphenyl-phosphine in decalin. The substitution does not go to completion in the presence of carbon monoxide and the kinetics of the forward and reverse reaction for each step have been studied. The unsubstituted complex undergoes direct attack by PPh3 either on the predominant anti-form or on the very reactive syn-form which is produced in the rate-determining anti-syn-isomerisation. The monosubstituted complex, which is also present mainly in the anti-form in equilibrium with a reactive syn-form, reacts with carbon monoxide through an SN2 mechanism, but a CO-dissociative mechanism is involved in its reaction with the bulkier PPh3. The bis(phosphine) complex so obtained is unable to undergo SN2 displacement even by carbon monoxide and must previously lose one molecule of phosphine. Relative rate constants for bimolecular attack on the co-ordinatively unsaturated intermediate by carbon monoxide and triphenylphosphine have been obtained. Equilibrium and activation parameters for these reactions are reported.