Os-III(N-2)Os-II complexes at the localized-to-delocalized, mixed-valence transition

On the basis of the appearance of v(N drop N) from 2007 to 2029 cm(-1) in C D3CN, oxidation states at Os in cis,cis-[(bpy)(2)(Cl)Os-III(N-2)Os-II(Cl)(b py)(2)](3+) (4), trans,trans-[(tpy)(Cl)(2)Os-III(N-2)Os-II(Cl)(2)(tpy)](+) (5), [(tpm)(Cl)(2)Os-III(N-2)Os-II(Cl)(2)(tpm)](+) (7), and [(Tp)(Cl)(2)Os- III(N-2)Os-II(Cl)(2)(Tp)](-) (8) are localized. For 5 this conclusion is re inforced by the crystal structure of the BF4- salt, which shows that the Os sites are unsymmetrical. Five narrow, solvent-independent bands appear in the near-infrared. They can be assigned to a combination of intervalence (I T) and interconfigurational d pi --> d pi transitions. From integrated band intensities the magnitudes of the resonance energies from electronic coupl ing across the mu-N-2 bridge vary from 1500 to 2500 cm(-1) Intramolecular O s-II --> Os-III electron transfer is rapid on the time scale for solvent re orientation, but a barrier still exists from low-frequency, coupled Os-Cl a nd Os-N vibrations.