OsIII(N₂)OsIIComplexes at the Localized-to-Delocalized, Mixed-Valence Transition

On the basis of the appearance of ν(N⋮N) from 2007 to 2029 cm - 1 in CD 3 CN, oxidation states at Os in cis,cis -[(bpy) 2 (Cl)Os III (N 2 )Os II (Cl)(bpy) 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 reinforced by the crystal structure of the BF 4 - 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 (IT) and interconfigurational dπ → dπ transitions. From integrated band intensities the magnitudes of the resonance energies from electronic coupling across the μ-N 2 bridge vary from 1500 to 2500 cm - 1 . Intramolecular Os II → Os III electron transfer is rapid on the time scale for solvent reorientation, but a barrier still exists from low-frequency, coupled Os−Cl and Os−N vibrations.