Electronic Structures of Homoleptic [Tris(2,2′-bipyridine)M]^<i>n</i> Complexes of the Early Transition Metals (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta; <i>n</i> = 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-<i>tert</i>-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]^<i>m</i> (<i>m</i> = 0, 1–, 2–, 3−), [Ti­(bpy)₃]^<i>n</i> (<i>n</i> = 1+, 0, 1–, 2–, 3−), [Zr­(bpy)₃]<i>^p</i>, and [Hf­(bpy)₃]<i>^p</i> (<i>p</i> = 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]<i>^m</i> 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 (bpy^2‑)^2‑ in the trianion. The same is true for the [Ti­(bpy)₃]^<i>n</i> series where the neutral complex contains 3­(bpy^•)^1‑ and the trianion 3­(bpy^2‑)^2‑ anions. Hence, the central ion always possesses a central Ti^III (d¹) ion that intramolecularly antiferromagnetically couples to any (bpy^•)^1‑ ligands present. In contrast, the central metal ions in the series [Zr­(bpy)₃]^<i>p</i> and [Hf­(bpy)₃]<i>^p</i> always possess a +IV oxidation state; hence, the dianions contain three (bpy^2‑)^2‑ ligands and yield an <i>S</i> = 0 ground state. The electronic structures of the neutral Nb and Ta analogues possessing <i>S</i> = ¹/₂ ground states are best described as [Nb^IV(bpy^2‑)₂(bpy)⁰)]⁰ and [Ta^V(bpy^•)­(bpy^2‑)₂]⁰, and their <i>S</i> = 0 monoanions as [Nb^IV(bpy^•)­(bpy^2‑)₂]^1‑ and [Ta^V(bpy^2‑)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