The Shortest Metal−Metal Bond Yet: Molecular and Electronic Structure of a Dinuclear Chromium Diazadiene Complex

Reduction of a diazadiene chromium halide complex, [(HLiPr)Cr(μ-Cl)]2 (1, HLiPr = bis(2,6-diisopropylphenyl)diazadiene), with KC8 gave a diamagnetic, bimetallic complex, (HLiPr)2Cr2 (2). Complex 2 has been structurally characterized by X-ray crystallography and consists of a Cr2 unit spanned by two diazadiene ligands. The very short Cr−Cr distance (1.8028(9) Å) 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-unrestricted DFT 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.