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Nearest- and Next-Nearest-Neighbor Ru(II)/Ru(III) Electronic Coupling in Cyanide-Bridged Tetra-Ruthenium Square Complexes.

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journal contribution
posted on 05.09.2011, 00:00 authored by Ju-Ling Lin, Chia-Nung Tsai, Sheng-Yi Huang, John F. Endicott, Yuan-Jang Chen, Hsing-Yin Chen
Electrochemical properties of cyanide-bridged metal squares, [Ru4]4+ and [Rh2–Ru2]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 [Ru4]4+ are ΔE1/2 = 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 ΔE1/2 = 77 mV and no MMCT absorption bands observed to accompany the oxidations of [Rh2–Ru2]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.