Analysing the chromium–chromium multiple bonds using multiconfigurational quantum chemistry

This Letter discusses the nature of the chemical bond between two chromium atoms in different di-chromium complexes with the metal atoms in different oxidation states. Starting with the Cr diatom, with its formally sextuple bond and oxidation number zero, we proceed to analyse the bonding in some Cr(I)–Cr(I) XCrCrX complexes with X varying from F, to Phenyl, and Aryl. The bond distance in these complexes varies over a large range: 1.65–1.83 Å and we suggest explanations for these variations. A number of di-chromium complexes with bond distances around or shorter than 1.80 Å have recently been synthesized and we study one of these complexes, Cr2(diazadiene)2 and show how the Cr–Cr bond order is related to the oxidation number and the ligand bonding, factors that are all involved in the determination of the short Cr–Cr bond length: 1.80 Å. The discussion is based on the use of multiconfigurational wave functions, which give a qualitatively correct description of the electronic structure in these multiply bonded systems.