Intervalence transfer at the localized-to-delocalized, mixed-valence transition in osmium polypyridyl complexes

The mixed-valence complexes [(bpy)(2)(Cl)Os-III(BL)Os-II(Cl)(bpy)(2)](3+) a nd [(tpy)(bpy)Os-III(BL)Os-II(bpy)(tpy)](5+) (bpy is bipyridine; tpy is 2,2 ':6',2"-terpyridine; BL is a bridging ligand, either 4,4'-bipyridine (4,4'- bpy) or pyrazine (pz)) have been prepared and studied by infrared and near- infrared measurements in different solvents. For BL = 4,4'-bpy, there is cl ear evidence for localized Os-II and Os-III oxidation states in the appeara nce of the expected two interconfigurational d pi -> d pi bands at Os-III a nd additional, broad absorption features in the near-infrared arising from intervalence transfer (IT) transitions. For [(bpy)(2)(Cl)Os(pz)Os(Cl)(bpy)( 2)](3+) and [(tpy)(bpy)Os(pz)Os(bpy)(tpy)](5+), unusually intense Il(pz) ba nds appear in the infrared at 1599 cm(-1) (epsilon = 2600 M-1 cm(-1)) for t he former and at 1594 cm(-1) (epsilon = 2020 M-1 cm(-1)) for the latter. Th ey provide an oxidation state marker and evidence for localized oxidation s tates. A series of bands appear in die near-infrared from 2500 to 8500 cm(- 1) that can be assigned to a combination of interconfigurational d pi -> d pi and IT transitions. In CD3CN, in the mid-infrared, bands arising from v( bpy) ring stretching modes from 1400 to 1500 cm(-1) are averaged for [(bpy) (2)(Cl)Os(pz)Os(Cl)(bpy)(2)](3+) Or significantly perturbed for [(tpy)(bpy) Os(pz)Os(bpy)(tpy)](5+) compared to electronically isolated Os-II and Os-II I complexes. The pyrazine-bridged complexes have properties that place them in a new class of mixed-valence molecules, Class II-III having properties associated with both Class II and Class III in the Robin and Day classifica tion scheme. The characteristic features of this class are that oxidation s tates are localized because of vibrational coupling but that solvent orient ational motions are uncoupled because of rapid intramolecular electron tran sfer.