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

New mixed-valence complexes of the type [(terpy)(bipy)Ru-II-L-Ru-III(N H3)(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) eth ylene) exhibit metal-to-metal (Ru-b(III) --> Ru-a(III); Ru-b = Ru bond ed to bipyridine, Ru-a = Ru bonded to ammine) charge transfer transiti ons in the visible region, due to the strong asymmetry of the redox si tes. Although the electronic coupling element of the pz-bridged comple x is higher than that of the BPE analogue, both complexes are consider ed partially delocalized (Robin and Day class II). From a comparison o f these data and those from closely related compounds, the distance de pendence of intramolecular electron transfer parameters has been deter mined over a range of metal-to-metal distances I from 5 to congruent t o 14 Angstrom, good correlations being obtained, the electronic coupli ng H-AB and the molar absorptivity epsilon(max) decreasing exponential ly with r. The bridging ligands appear to behave as electronic pi medi ators with intermediate conducting properties (beta = 0.40 Angstrom(-1 )). The reorganization energy 3,increases with r, but for r > 9 Angstr om, the intramolecular electron transfer back reactions Ru-a(II) --> R u-b(III) fall in the barrierless regime, where the nuclear factor show s small distance dependence. In order to slow charge recombination aft er photoexcitation, it may be possible to combine more asymmetric redo x sites, and by manipulation of the distance between them, generate in termediate values of H-AB, thereby causing Marcus ''inverted'' behavio ur.