Ligand to metal charge transfer transitions in Ru(III) and Os(III) complexes of substituted 2,2'-bipyridines

The variations in the energy and intensity of the low-energy LMCT transitions have been examined in the complexes [M(X2-bpy)3]3+ and [M(bpy)n(X2-bpy)3-n]3+ (M = Ru or Os, X = Me, OMe, NH2 and NMe2). In the homo tris-chelates the energy of the low-energy LMCT transition falls within a narrow range of 1.80 ± 0.1V for Ru(III) and 2.20 ± 0.1 V for Os(III). The LMCT transition occurs at much lower energy in mixed ligand complexes. A quantitative analysis as a function of donor strength and Hammett substituent constants is presented.