ENHANCED INTENSITIES OF THE LIGAND-TO-METAL CHARGE-TRANSFER TRANSITIONS IN RU(III) AND OS(III) COMPLEXES OF SUBSTITUTED BIPYRIDINES

Ru(III) and Os(III) complexes of 2,2'-bipyridine carrying electron-donor substituents show fairly intense ligand-to-metal charge-transfer (LMCT) absorption band(s) in the red/near-IR region. The intensity of the LMCT band increases with increasing effective donor strength of the substituted bpy ligand. The variations in the energies and intensities of these LMCT bands has been examined in twenty complexes of the type 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 trischelates the energy of the low-energy LMCT transition falls within a narrow range of 1.80 +/- 0.1 V for Ru(III) and 2.20 +/- 0.1 V for Os(III). The energy of the LMCT transition is much lower in mixed-ligand complexes. A quantitative analysis as a function of donor strength and Hammett substituent constants is presented.