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](PF6) (M = Mo, X = Cl and M = W, X = F)

Abstract

The electronic structures of a series of so-called "low-valent" transition metal complexes [M((Me)bpy)3](0) and [M(tpy)2](0) ((Me)bpy = 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 [Ti(IV)(tpy(2-))2](0) (S = 0, 6), [V(IV)(tpy(2-))2] (S = 1/2, 7), [Ti(III)((Me)bpy(•))3](0) (S = 0, 1), [V(II)((Me)bpy(•))2((Me)bpy(0))](0) (S = 1/2, 2), and [Mo(III)((Me)bpy(•))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 [Mo(III)(bpy(0))2Cl2]Cl·2.5CH3OH (S = 3/2, 13), and seven-coordinate species [Mo(IV)F((Me)bpy(•))2((Me)bpy(0))](PF6) (S = 0, 5), [Mo(IV)Cl(tpy(•))2](PF6)·CH2Cl2 (S = 0, 11), and [W(V)F(tpy(•))(tpy(2-))](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 C(py)-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.