Abstract

The electronic structures of the complexes [M(tbpy)3]0,1- (M = Nb, Ta; tbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) have been investigated using a combination of UV–vis spectroscopy, EPR spectroscopy, and XAS. Furthermore, the crystal structure of [Na(THF)5][Ta(tbpy)3] has been determined. These studies were supplemented by density functional theory (DFT) and the calculations extended to include the series [Y(bpy)3]m (m = 0, 1–, 2–, 3−), [Ti(bpy)3]n (n = 1+, 0, 1–, 2–, 3−), [Zr(bpy)3]p, and [Hf(bpy)3]p (p = 0, 1–, 2−). This has allowed us to define the correct electronic structures of these early transition metal tris(2,2′-bipyridine) complexes. It is shown that in the [Y(bpy)3]m series the central ion possesses an invariant +III oxidation state and that the three successive one-electron redox processes that comprise the series are solely ligand-based, yielding three (bpy•)1- radical anions in the neutral complex through to three diamagnetic dianions (bpy2-)2- in the trianion. The same is true for the [Ti(bpy)3]n series where the neutral complex contains 3(bpy•)1- and the trianion 3(bpy2-)2- anions. Hence, the central ion always possesses a central TiIII (d1) ion that intramolecularly antiferromagnetically couples to any (bpy•)1- ligands present. In contrast, the central metal ions in the series [Zr(bpy)3]p and [Hf(bpy)3]p always possess a +IV oxidation state; hence, the dianions contain three (bpy2-)2- ligands and yield an S = 0 ground state. The electronic structures of the neutral Nb and Ta analogues possessing S = 1/2 ground states are best described as [NbIV(bpy2-)2(bpy)0)]0 and [TaV(bpy•)(bpy2-)2]0, and their S = 0 monoanions as [NbIV(bpy•)(bpy2-)2]1- and [TaV(bpy2-)3]1-. The central metal ion in the Nb series maintains a +IV oxidation state, while in the Ta series the central metal ion displays a +V oxidation state throughout.