Mechanism of inner-sphere electron transfer via charge-transfer (precursor) complexes. Redox energetics of aromatic donors with the nitrosonium acceptor

Spontaneous formation of colored (1:1) complexes of various aromatic donors (ArH) with the nitrosonium acceptor (NO+) is accompanied by the appearance of two new (charge-transfer) absorption bands in the UV-vis spectrum. IR s pectral and X-ray crystallographic analyses of the [ArH,NO+] complexes reve al their inner-sphere character by the ArR/NO+ separation that is substanti ally less than the van der Waals contact and by the significant enlargement of the aromatic chromophore. The reversible interchange between such an in ner-sphere complex [ArH,NO+] and the redox product (ArH+. + NO.) is quantit atively assessed for the first time to establish it as the critical interme diate in the overall electron-transfer process. Theoretical formulation of the NO+ binding to ArH is examined by LCAO-MO methodology sufficient to all ow the unambiguous assignment of the pair of diagnostic (UV-vis) spectral b ands. The MO treatment also provides quantitative insight into the high deg ree of charge-transfer extant in,these inner-sphere complexes as a function of the HOMO-LUMO gap for the donor/acceptor pair. The relative stabilizati on of [ArH,NO+] is traced directly to the variation in the electronic coupl ing element H-AB, which is found to be substantially larger than the reorga nization energy (lambda /2). In Sutin's development of Marcus-Hush theory, this inequality characterizes a completely delocalized Class III complex (w hich occupies a single potential well) according to the Robin-Day classific ation. The mechanistic relevance of such an unusual (precursor) complex to the inner-sphere mechanism for organic electron transfer is discussed..