The shortest metal-metal bond yet: Molecular and electronic structure of a dinuclear chromium diazadiene complex

Reduction of a diazadiene chromium halide complex, [(L-H(iPr))Cr((sic)-CI)](2) (1,L-H(iPr) = bis(2,6-diisopropylphenyl)diazadiene), with KC8 gave a diamagnetic, bimetallic complex, (L-H(iPr))(2)Cr-2 (2). Complex 2 has been structurally characterized by X-ray crystallography and consists of a Cr-2 unit spanned by two diazadiene ligands. The very short Cr-Cr distance (1.8028(9) angstrom) and low formal oxidation state of the Cr atoms suggest that the Cr-Cr bond order may be greater than 4. Spin-restricted and spin-u n restricted DFF calculations on a model complex both converge on a singlet spin state and accurately reproduce the metric parameters of 2. The calculations show high-order metal-metal bonding along with extensive delocalization over the diazadiene ligands. Natural resonance theory analysis shows population of resonance structures that contain quintuple bonds, and natural bond order analysis gives a bond order of 4.28 for the Cr-Cr bond.