Electronic Structures of Homoleptic [Tris(2,2′-bipyridine)M]n Complexes of the Early Transition Metals (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta; n = 1+, 0, 1–, 2–, 3−): An Experimental and Density Functional Theoretical Study

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.