Molecular and Electronic Structures of Six-Coordinate “Low-Valent” [M(^Mebpy)₃]⁰ (M = Ti, V, Cr, Mo) and [M(tpy)₂]⁰ (M = Ti, V, Cr), and Seven-Coordinate [MoF(^Mebpy)₃](PF₆) and [MX(tpy)₂](PF<sub>6</sub

The electronic structures of a series of so-called “low-valent” transition metal complexes [M­(Mebpy)3]0 and [M­(tpy)2]0 (Mebpy = 4,4′-dimethyl-2,2′-bipyridine and tpy = 2,2′,6′,2″-terpyridine) have been determined using a combination of X-ray crystallography, magnetochemistry, and UV–vis–NIR spectroscopy. More specifically, the crystal structures of the long-known complexes [TiIV(tpy2–)2]0 (S = 0, 6), [VIV(tpy2–)2] (S = 1/2, 7), [TiIII(Mebpy•)3]0 (S = 0, 1), [VII(Mebpy•)2(Mebpy0)]0 (S = 1/2, 2), and [MoIII(Mebpy•)3]0 (S = 0, 4) have been determined for the first time. In all cases, the experimental results confirm the electronic structure assignments that we ourselves have recently proposed. Additionally, the six-coordinate complex [MoIII(bpy0)2Cl2]­Cl·2.5CH3OH (S = 3/2, 13), and seven-coordinate species [MoIVF­(Mebpy•)2(Mebpy0)]­(PF6) (S = 0, 5), [MoIVCl­(tpy•)2]­(PF6)·CH2Cl2 (S = 0, 11), and [WVF­(tpy•)­(tpy2–)]­(PF6)·CH2Cl2 (S = 0, 12) have been synthesized and, for the first time, crystallographically characterized. Using the resulting data, plus that from previously published high-resolution X-ray structures of analogous compounds, it is shown that there is a linear correlation between the average Cpy–C′py bond distances in these complexes and the total charge (n) of the ligands, {(bpy)3}n and {(tpy)2}n. Hence, an assignment of the total charge of coordinated bpy or tpy ligands and, by extension, the oxidation state of the central metal ion can reliably be made on the basis of X-ray crystallography alone. In this study, the oxidation states of the metal ions range from +II to +V and in no case has an oxidation state of zero been validated. It is, therefore, highly misleading to use the term “low-valent” to describe any of the aforementioned neutral complexes.