DISTANCE DEPENDENCE OF INTRAMOLECULAR ELECTRON-TRANSFER PARAMETERS IN MIXED-VALENCE ASYMMETRIC COMPLEXES OF RUTHENIUM

New mixed-valence complexes of the type [(terpy)(bipy)Ru-II-L-Ru-III(NH3)(5)](5+) (terpy = 2,2':6',2''-terpyridine, bipy = 2,2'-bipyridine) with L = pz and BPE (pz = pyrazine; BPE = trans-1,2-bis(4-pyridyl) ethylene) exhibit metal-to-metal (Ru-b(III) --> Ru-a(III); Ru-b = Ru bonded to bipyridine, Ru-a = Ru bonded to ammine) charge transfer transitions in the visible region, due to the strong asymmetry of the redox sites. Although the electronic coupling element of the pz-bridged complex is higher than that of the BPE analogue, both complexes are considered partially delocalized (Robin and Day class II). From a comparison of these data and those from closely related compounds, the distance dependence of intramolecular electron transfer parameters has been determined over a range of metal-to-metal distances I from 5 to congruent to 14 Angstrom, good correlations being obtained, the electronic coupling H-AB and the molar absorptivity epsilon(max) decreasing exponentially with r. The bridging ligands appear to behave as electronic pi mediators with intermediate conducting properties (beta = 0.40 Angstrom(-1)). The reorganization energy 3,increases with r, but for r > 9 Angstrom, the intramolecular electron transfer back reactions Ru-a(II) --> Ru-b(III) fall in the barrierless regime, where the nuclear factor shows small distance dependence. In order to slow charge recombination after photoexcitation, it may be possible to combine more asymmetric redox sites, and by manipulation of the distance between them, generate intermediate values of H-AB, thereby causing Marcus ''inverted'' behaviour.