Infrared study of the Class II/Class III boundary in mixed-valence dinuclear ruthenium complexes

The cyanamide stretching frequencies of the mixed-valence complexes [{Ru(NH3)5}2(μ-L)]3+, trans,trans-[{Ru(NH3)4(py)}2(μ-L)] 3+ (py = pyridine), and mer,mer-[{Ru(NH3)3(bpy)}2(μ-L)] 3+ (bpy = 2,2′-bipyridine), where L is the 1,4-dicyanamidobenzene dianion and its substituted derivatives in nitromethane, acetonitrile, and dimethyl sulfoxide, were examined to determine the effects of inner- and outer-sphere perturbation on electron delocalization. The solvent-dependent infrared spectra of [{Ru(NH3)5}2(μ-Me2dicyd)] 3+, where Me2dicyd2- is the 2,5-dimethyl-1,4-dicyanamidobenzene dianion, yield evidence for a transformation from localized to delocalized behavior and confirm the effect of outer-sphere perturbation on the mixed-valence state. The IR spectrum of trans,trans-[{Ru(NH3)4(py)}2(μ-Me 2dicyd)]3+ in acetonitrile is consistent with Class III properties, and this complex's properties can be regarded as benchmarks for delocalization in our systems. It is shown that this complex obeys the general condition for delocalization in symmetric mixed-valence complexes, 2H = λ, and possesses an experimental free energy of resonance exchange ΔGr′ = 1250 cm-1 and resonance exchange integral H = 3740 cm-1