Element Misidentification in X‑ray Crystallography: A Reassessment of the [MCl2(diazadiene)] (M = Cr, Mo, W) Series

A series
of reports describing the syntheses and structures of
[MCl2(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 [MCl2(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 [ZnCl2(MesdadMe)] (MesdadMe = 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 [WCl2(diazadiene)]
with MesdadMe and [WBr4(MeCN)2] in lieu of [WCl4(MeCN)2] is shown
to produce [ZnBr2(MesdadMe)], which
has also been characterized by X-ray diffraction. It is concluded
that the unusual putative 12-electron [MCl2(diazadiene)]
(M = Cr, Mo, W) complexes are in all cases the corresponding [ZnCl2(diazadiene)] complexes, Zn having been commonly employed
as reducing agent in their synthesis.

一系列报告对[MCl2(diazadiene)]（M = Cr, Mo, W）络合物的合成与结构进行了重新评估，这一评估基于已知的低价VI族金属络合物的化学性质、晶体学趋势如M–Cl键长和晶胞体积，以及计算得到的金属-配体键长。晶体学数据和计算结果与这些物种作为第二或第三周期过渡金属络合物的假设不符。对[MCl2(diazadiene)]（M = Mo, W）已发表的晶体学信息文件的回顾表明，金属原子被不适当地处理，使用了部分占有率因子（Mo：0.775；W：0.4005和0.417），其影响表现为更轻元素的行为和与金属原子正确身份相符的更好精修。通过X射线衍射对[ZnCl2(MesdadMe)]（MesdadMe = 1,4-双(2,4,6-三甲基苯基)-2,3-二甲基-1,4-二氮杂-1,3-丁二烯）的故意合成和表征被报道。使用相同的第二或第三周期金属组合以及抵消的部分占有率因子对该结构的精修显示，最终精修统计量基本上与采用M = Zn在占有率1.00的正确模型一致。使用已发表的合成[WCl2(diazadiene)]的方法，以MesdadMe和[WBr4(MeCN)2]替代[WCl4(MeCN)2]，显示出可以产生[ZnBr2(MesdadMe)]，该化合物也已被X射线衍射表征。结论是，异常的拟12电子[MCl2(diazadiene)]（M = Cr, Mo, W）络合物在所有情况下都是相应的[ZnCl2(diazadiene)]络合物，因为Zn在其合成中通常被用作还原剂。