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((t)bpy)(3)](0,1). (M = Nb, To; (t)bpy = 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((t)bpy)(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(.))(I). 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)](n) 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 (d(1)) 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 (bpy(2.))(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 [Nb-IV(bpy(2-))(2)(bpy)(0))](0) and [Ta-V(bpr)(bpy(2-))(2)](0), and their S = 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.