Probing mixed valence in a new tppz-bridged diruthenium(III,II) complex {(mu-tppz)[Ru(bik)Cl]2}3+ (tppz=2,3,5,6-tetrakis(2-pyridyl)pyrazine, bik=2,2'-bis(1-methylimidazolyl)ketone): EPR silence, intervalence absorption, and nu(CO) line broadening

The complex dication of the diruthenium(II) compound {(mu-tppz)Ru(bik)Cl]2}(ClO4)2 can be oxidized and reduced in two one-electron steps each. In CH3CN/0.1 M Bu4NPF6, the odd-electron intermediates{(mu-tppz)Ru(bik)Cl]2}n+, n=1 and 3, have comproportionation constants of 7x10(8) and 1x10(5), respectively. Both exhibit near-infrared absorptions, in the case of n=3 the 1640 nm band (epsilon=1200 M-1 cm-1, Deltanu1/2=1560 cm-1) is attributed to an intervalence charge-transfer transition. While the mixed-valent intermediate (n=3) is EPR silent even at 4 K, the n=1 form shows g(parallel) 2.005 and g( perpendicular) 1.994 at that temperature, signifying a diruthenium(II) complex of the tppz*- radical anion. The variation of energy and intensity of nuCO and of the ring vibration band around 1590 cm-1 has been monitored not only for {(mu-tppz)Ru(bik)Cl]2}n+, n=0-4, but also for the mononuclear {(tppz)Ru(bik)Cl}n+, n=0-2. In the dinuclear complex the carbonyl stretching bands of the spectator ligand bik are shifted by about 15 cm-1 on each one-electron-transfer step, increasing with the positive charge. The mixed-valent {(mu-tppz)Ru(bik)Cl]2}3+ shows a perceptibly broader nuCO band, suggesting incomplete valence averaging (partial localization).