Zur Kristallstruktur des Bis(allyl)‐chrom(II) [(C₃H₅)₂Cr]₂

Chromium Compounds

“…It is also reported that on addition of excess THF the dinuclear (LiTHF) 4 Another molecule having Cr-C bonds is the allyl molecule, Cr 2 (C 3 H 5 ) 4 , which is one of the earliest Cr 2 4+ compounds to be structurally characterized. 101,102 The planes of the allyl groups all lie parallel to the Cr-Cr bond. This pyrophoric compound, which has a molybdenum analog, is easily made by reaction of CrCl 3 with an excess of allyl Grignard, or by reduction of Cr(C 3 H 5 ) 3 with C 2 H 5 Li.…”

Contributions of Erhard Kurras to Organochromium Chemistry – From a Historical Perspective

“…Quadruple and other multiple metal‐to‐metal bonds were investigated for several metals [12a] . A search for published structural data of chromium(II) compounds with a Cr−Cr quadruple bond using the Cambridge Structural Database (CSD version 5.42) and SciFinder yielded 298 structures as of 8/22/2021 (1953: 1, [12b] 1967: 1, [13d] 1969: 1, [13c] 1970: 2, [12c,13a] 1971: 1, [12f,13b] 1978: 12, 1979–today: 280).…”

“…Cotton wrote in 1967, “Is there actually a Cr−Cr bond? The structural parameters (i. e., d(Cr−Cr)=2.64 Å) indicate clearly that there is no bond.” [12a] Clearly, this statement was derived based on a molecular structure determination for Cr 2 (CH 3 COO) 4 ⋅ 2H 2 O with a published very long Cr−Cr bond distance of 2.64 Å, so the Cr−Cr interaction was characterized by Cotton as very weak, however, already in 1967 Albrecht and Stock published X‐ray data of Kurras's [(C 3 H 5 ) 2 Cr] 2 with a Cr−Cr distance of 1.98 Å [13d] . To the best of our knowledge, this X‐ray study of Kurras′ crystals of [(C 3 H 5 ) 2 Cr] 2 represents the first accurate determination of a chromium‐chromium quadruple bond and it is incomprehensible why Cotton ignored this work.…”

“…Kurras had no direct access to modern single crystal structure analysis. For this reason, he sent his crystals to working groups that had these facilities, e. g. Albrecht, Krausse, and even Cotton [13a,b,d,14] . This led to the fact that almost all publications with structural data on Kurras's chromium(II) compounds were published without him, he was ”only“ thanked.…”

“…Ultimately, all of Kurras′ novel dichromium(II) compounds were studied by X‐ray diffraction and indeed, they all possess very strong Cr−Cr interactions and can be described via a Cr−Cr quadruple bond [13] . For example, tetrallyldichromium(II) compounds, [13c–e] and the later synthesized similar alkyl and phenyl compounds feature also very strong Cr−Cr bonds, [1d,e] or [Li 4 Cr 2 (CH 3 ) 8 (C 4 H 8 O) 4 ], [13a] [Li 4 Cr 2 (C 4 H 8 ) 4 (C 4 H 8 O) 4 ], [13b] and tetraallyldichromium(II) Cr 2 (C 3 H 5 ) 4 [13c–e] In 1978, four years after their synthesis by Kurras and co‐workers in 1974, the dinuclear complexes of Cr(II) and Mo(II) with bridging dimethylphosphonium bis(methylide) ligands (Figure 3C) were studied by Cotton and co‐workers, showing a “supershort” Cr−Cr quadruple bond of 1.895(3) Å [14a,b] …”