Short and Supershort Cr–Cr Distances: A Vanishing Borderline Between Metal–Metal Bonds, Magnetic Couplings and Ligand Artifacts

The structural features of three classes of Cr(II) compounds (alkoxides, amides, aryls) are critically reviewed with the aim of investigating the role of ligand bridging interactions in the formation of short Cr–Cr quadruple bonds. A systematic failure of Cr–Cr bond formation is observed with either monodentate or bidentate ligands. Fairly short Cr–Cr contacts are easily elongated and dinuclear structures cleaved by simply coordinating a molecule of solvent either to chromium or to the alkali cation present in anionic chromates. Conversely, extremely short Cr–Cr contacts are almost invariably formed when particular bridging ligands with a three-center chelating geometry and the electronic configuration of an allylic system have been used. Comparison of the magnetic properties of two series of Cr(II) aryls and pyrrolyls, where the three center chelating geometries of the ligands have been progressively modified, indicated that three-center chelating ligands are able to determine the dimeric aggregation, the magnetic properties of chromium and ultimately the length of Cr–Cr contacts.