Element Misidentification in X‑ray Crystallography: A Reassessment of the [MCl<sub>2</sub>(diazadiene)] (M = Cr, Mo, W) Series

A series of reports describing the syntheses and structures of [MCl<sub>2</sub>(diazadiene)] (M = Cr, Mo, W) complexes is reassessed in the context of known chemistry of low-valent Group VI metal complexes, crystallographic trends such as M–Cl bond lengths and unit cell volumes, and calculated metal–ligand bond lengths. Crystallographic data and computational results are inconsistent with any of these species being second or third row transition metal complexes. A review of the crystallographic information files accompanying the [MCl<sub>2</sub>(diazadiene)] (M = Mo, W) published structures reveals that the metal atoms were inappropriately treated with partial site occupancy factors (0.775 for Mo; 0.4005 and 0.417 for W), the effect of which was to manifest lighter-element behavior and better refinement in accord with the metal atoms’ correct identity. A deliberate synthesis and characterization by X-ray diffraction of [ZnCl<sub>2</sub>(<sup>Mes</sup>dad<sup>Me</sup>)] (<sup>Mes</sup>dad<sup>Me</sup> = 1,4-bis­(2,4,6-trimethylphenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene) are reported. Refinement of this structure with the same combination of second or third row metal and offsetting partial site occupancy is shown to provide final refinement statistics essentially the same as with the correct model employing M = Zn at site occupancy 1.00. Use of the published method for synthesis of [WCl<sub>2</sub>(diazadiene)] with <sup>Mes</sup>dad<sup>Me</sup> and [WBr<sub>4</sub>(MeCN)<sub>2</sub>] in lieu of [WCl<sub>4</sub>(MeCN)<sub>2</sub>] is shown to produce [ZnBr<sub>2</sub>(<sup>Mes</sup>dad<sup>Me</sup>)], which has also been characterized by X-ray diffraction. It is concluded that the unusual putative 12-electron [MCl<sub>2</sub>(diazadiene)] (M = Cr, Mo, W) complexes are in all cases the corresponding [ZnCl<sub>2</sub>(diazadiene)] complexes, Zn having been commonly employed as reducing agent in their synthesis.