Reactivity of cyclopentadienyl and indenyl alkylcarbonyliron(II) complexes. eta(5) species as key intermediates in the migratory insertion of carbon monoxide

The reactions of the complexes [Fe(eta(5)-L)(CO)(2)R] (L = C5H5 or C9H7, R = Me or CHMe(2)) with PR'(3) (PMe(2)Ph, PMePh(2) and PPh(3)) have been studied in toluene and tetrahydrofuran (thf) by UV/VIS and IR spectroscopy. The products of alkyl migratory insertion [Fe(eta(5)-C5H5)(CO)(COR)(PR'(3))] are formed via an associative mechanism, involving (i) a rapid pre-equilibrium (K) between complex and phosphine and (ii) rate-determining alkyl migration (k(2)), in analogy to the mechanism previously established for [Fe(eta(5)-C9H7)(CO)(2)R]. In the intermediate molecular complexes {[Fe(eta(5)-L)(CO)(2)R], phosphine} the compounds [Fe(eta(5)-C5H5)(CO)(2)R] bind to phosphine less efficiently than do the indenyl analogues and react one order of magnitude more slowly. Reaction rates are similar in thf and in toluene, in agreement with the proposal that the alkyl migration is not solvent assisted.