Reaction mechanisms of metal–metal bonded carbonyls. Part III. Some reactions of rhenium decacarbonyl

Abstract

The substitution reaction of rhenium decacarbonyl with triphenylphosphine, its oxidation reactions with iodine and oxygen, and its thermal decomposition in the absence of any reagent, have been studied kinetically between 130 and 150° in decalin. The kinetic behaviour is generally good, and the reactions show a strong resemblance to the corresponding ones of manganese decacarbonyl with the main exception that little, if any, substitution occurs by a path involving the easily reersible dissociation of carbon monoxide as a rate-determining step. A general reaction scheme for manganese and rhenium decacarbonyl is suggested in which an important rate-determining step is the formation of a species (OC)₅M–C(:O)–M(CO)₄ by a ‘metal-migration’ reaction.

The activation enthalpies required to form this type of intermediate in systems containing Co–Co, Mn–Mn, and Re–Re bonds are in qualitiative agreement with both force constant data and dissociation energies estimated from mass spectrometry, in that it can be inferred that the dissociation energies of the metal–metal bonds increase along the series.

The reaction with iodine follows a pseudo-first-order rate law: k_obs=k_a+k_b[l₂]. k_a is only approximately determinable, but it is essentially the same as the rate of the metal-migration reaction. The activation parameters of the bimolecular path are very different from those for the corresponding reaction of manganese decacarbonyl.