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(NH 3 ) 5 2+, and the acceptor = Ru(NH 3 ) 5 3+ . The electronic coupling (and electron delocalization) between the donor and the central atom (M) of the bridging ligand varies from H DL ≈ 10 3 to ∼3 × 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.