Abstract
Valence‐state distributions in the ruthenium quinonoid (L) frameworks of [Ru(trpy)(Cl)(L^1^)]ClO~4~ (1‐ClO~4~) and [Ru(trpy)(Cl)(L^2^)]ClO~4~ (2‐ClO~4~) (L^1^ = o‐iminobenzoquinone, L^2^ = o‐diiminobenzoquinone, and trpy = 2,2′:6′,2″‐terpyridine) have been examined by structural, spectroelectrochemical, and density functional studies. The structural data, in corroboration with the DFT‐calculated bond lengths, suggest that the primary valence formulation of 1^+^ and 2^+^ is a spin‐coupled singlet configuration of [Ru^III^(trpy)(Cl)(L~Sq~)]^+^ with a minority contribution from diamagnetic [Ru^II^(trpy)(Cl)(L~Q~)]^+^. Consequently, the closely spaced successive two oxidation processes of 1^+^ and 2^+^ can be assigned to Ru^III^→Ru^IV^ and L~Sq~→L~Q~, which involve the HOMO and HOMO–3 levels, respectively. The one‐electron‐oxidized species 1^2+^ and 2^2+^ display sharp EPR signals with g values of 2.011 and 2.014 at 77 K, respectively. The free radical EPR signal (g ≈ 2.0) of the one‐electron‐reduced species 1 or 2 signifies the preferential involvement of the ruthenium‐based orbitals in the first reduction process to yield [Ru^II^(trpy)(Cl)(L~Sq~)], although the LUMO is calculated to be a mixture of dπ(Ru) (≈24 %) and π*(L) (≈ 70 %). The subsequent second (1^–^/2^–^) and third (1^2–^/2^2–^) reduction steps in each case are associated simply with the terpyridyl‐based orbitals (≥90 %). The lowest energy charge‐transfer transitions of 1^+^ and 2^+^ at 556 and 509 nm are predicted to be HOMO → LUMO+1 and HOMO–1 → LUMO+1 transitions, respectively. In the successive oxidations 1^+^/2^+^ → 1^2+^/2^2+^ → 1^3+^/2^3+^ the lowest energy charge‐transfer transitions undergo a blue shift with a substantial reduction in intensity. The lowest energy charge‐transfer transitions, however, are red shifted with a reduction in intensity on going from (1^+^/2^+^) to 1/2. The origin of the transitions in the 1^2+^/2^2+^ and 1/2 systems is predicted by TDDFT analysis. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)