NUCLEOPHILIC-ADDITION VERSUS ELECTRON-TRANSFER IN CARBONYLMETALLATE SALTS - DONOR-ACCEPTOR INTERACTIONS IN THE PRECURSOR ION-PAIRS

The isostructural pentacarbonylmetallate anions M(CO)(5)(-) (M=Mn and Re) react with a series of N-methylpyridinium cations (Py+) to yield p roducts of nucleophilic addition [NA=Py-M(CO)(5)] or of one-electron r edox reaction [ET=Py-.+M(CO)5(.)]. The partitioning of the reaction al ong the two reaction pathways is controlled by steric factors and the electronic structure of the pyridinium cation, with cations which form stable, delocalized radicals favoring the ET pathway. The central met al also plays a role in determining the stoichiometry, and the NA path way is favored by the rhenate anion and ET by the manganate analogue. Rates of both reactions correlate with the driving force for electron transfer, and the differing reaction pathways are not distinguished on the basis of linear free energy relations, previously discussed by Bo rdwell and co-workers. The contact ion pair [Py+,M(CO)(5)(-)] is ident ified as the critical precursor for both electron transfer and nucleop hilic coupling. Based on these observations, it is proposed that the r ates and mechanisms of these interionic reactions are controlled by do nor-acceptor bonding in the transition states, which in turn is direct ly related to the charge-transfer interactions extant in the ion-pair intermediate. (C) 1997 by John Wiley & Sons, Ltd.