Nearest- and next-nearest-neighbor Ru(II)/Ru(III) electronic coupling in cyanide-bridged tetra-ruthenium square complexes.

Electrochemical properties of cyanide-bridged metal squares, [Ru(4)](4+) and [Rh(2)-Ru(2)](6+), clearly demonstrate the role of the nearest (NN) metal moiety in mediating the next-nearest neighbor (NNN) metal-to-metal electronic coupling. The differences in electrochemical potentials for successive oxidations of equivalent Ru(II) centers in [Ru(4)](4+) are ΔE(1/2) = 217 mV and 256 mV and are related to intense, dual metal-to-metal-charge-transfer (MMCT) absorption bands. This contrasts with a small value of ΔE(1/2) = 77 mV and no MMCT absorption bands observed to accompany the oxidations of [Rh(2)-Ru(2)](6+). These observations demonstrate NN-mediated superexchange mixing by the linker Ru of NNN Ru(II) and Ru(III) moieties and that this mixing results in a NNN contribution to the ground state stabilization energy of about 90 ± 20 meV. In contrast, the classical Hush model for mixed valence complexes with the observed MMCT absorption parameters predicts a NNN stabilization energy of about 6 meV. The observations also indicate that the amount of charge delocalization per Ru(II)/Ru(III) pair is about 4 times greater for the NN than the NNN couples in these CN-bridged complexes, which is consistent with DFT modeling. A simple fourth-order secular determinant model is used to describe the effects of donor/acceptor mixing in these complexes.