What does "through-bond coupling" mean? observations on simple donor-acceptor systems

A series of covalently linked, transition-metal donor/acceptor complexes are described in which the net donor-acceptor coupling matrix element, H-DA, is independent of the extent of coupling between the donor and the bridging ligand. The bridging ligand in these complexes is a transition-metal dicyano complex with a tetraaza aliphatic nonbridging ligand, cis- or trans-M(MCL)(CN)(2)(+) for M = Rh(III), Co(III), or Cr(III), donor = RU(NH3)(5)(2+), and the acceptor = Ru(NH3)(5)(3+). The electronic coupling land electron delocalization) between the donor and the central atom (M) of the bridging ligand varies from H-DL approximate to 10(3) to similar to 3 x 10(3) cm(-1) through the series of M(MCL)(CN)(2)(+)-bridged complexes, and this variation has an effect on the energy of the Ru(II)/Ru(III) CT absorption maximum, which is expected from perturbational mixing of these electronic states. However, the usually correlated superexchange contribution to H-DA is not observed and appears to be less than about 10% of the contribution predicted. This is in contrast to observations on related complexes with pyridyl-type bridging ligands. The unusual behavior can be a consequence of the dependence of D/A electronic coupling on the CN- vibrational distortions and the mixing of the two Ru(II)/Ru(III) electron-transfer states with the BL state promoted by in-phase and out-of-phase combinations of CN- stretches. Such an approach predicts very little superexchange coupling when there is little electron delocalization onto the bridging ligand and requires that H-DA be a strong function of the electron-transfer coordinates.