The ligand-based quintuple bond-shortening concept and some of its limitations.

Herein, the ligand-based concept of shortening quintuple bonds and some of its limitations are reported. In dichromium-diguanidinato complexes, the length of the quintuple bond can be influenced by the substituent at the central carbon atom of the used ligand. The guanidinato ligand with a 2,6-dimethylpiperidine backbone was found to be the optimal ligand. The reduction of its chromium(II) chloride-ate complex gave a quintuply bonded bimetallic complex with a Cr-Cr distance of 1.7056 (12) Å. Its metal-metal distance, the shortest observed in any stable compound yet, is of essentially the same length as that of the longest alkane C-C bond (1.704 (4) Å). Both molecules, the alkane and the Cr complex, are of remarkable stability. Furthermore, an unsupported Cr(I) dimer with an effective bond order (EBO) of 1.25 between the two metal atoms, indicated by CASSCF/CASPT2 calculations, was isolated as a by-product. The formation of this by-product indicates that with a certain bulk of the guanidinato ligand, other coordination isomers become relevant. Over-reduction takes place, and a chromium-arene sandwich complex structurally related to the classic dibenzene chromium complex was observed, even when bulkier substituents are introduced at the central carbon atom of the used guanidinato ligand.

在此，报道了基于配体缩短五重键的概念及其一些局限性。在二铬 - 二胍基配合物中，五重键的长度会受到所用配体中心碳原子上取代基的影响。发现具有2,6 - 二甲基哌啶骨架的胍基配体是最佳配体。其氯化铬（II）酸根配合物被还原后得到一个五重键合的双金属配合物，铬 - 铬距离为1.7056(12) Å。其金属 - 金属距离是在任何稳定化合物中所观察到的最短距离，与最长的烷烃C - C键（1.704(4) Å）的长度基本相同。烷烃和铬配合物这两种分子都具有显著的稳定性。此外，通过CASSCF/CASPT2计算表明，作为副产物分离出了一种在两个金属原子之间有效键级（EBO）为1.25的无支撑铬（I）二聚体。这种副产物的形成表明，当胍基配体具有一定的体积时，其他配位异构体变得相关。发生了过度还原，并且观察到一种在结构上与经典的二苯铬配合物相关的铬 - 芳烃夹心配合物，即使在所用胍基配体的中心碳原子上引入了更大体积的取代基时也是如此。