Valence delocalization despite weak metal-metal coupling in a bis(organoosmium(III,II)) complex with a pyrazine bridge

Carbonyl vibrational spectroelectrochemistry of {(mu-pz)[Os((PPr3)-Pr-i)(2)(CO)(H)Cl](2)}(0/+) in dichloromethane reveals valence delocalization of the mixed-valent state despite relatively weak metal-metal coupling, as evident from the comproportionation constant K-c = 10(4.3) and the intervalence charge-transfer band at 1705 nm (epsilon = 1250 M-1 cm(-1), Delta upsilon(1/2) = 3700 cm(-1)). The rather low charge and nonpolar medium, i.e., the absence of valence trapping by counterions or solvent molecules, favor this particular situation.