Binuclear complexes of all first row 3d metals (Sc to Zn) with a cyclic N4-tetradentate ligand: metal-metal bond strengths and bond orders

Metal-metal (MM) bonds in binuclear complexes M₂L₂ (L = tetradentate ligand) for the first row transition metals are studied using DFT to design novel complexes which mimic lantern complexes having four bidentate ligands. Two orientations for the ligand array are applied − orthogonal (orth) and lateral (lat). Ground states for all complexes are identified from several low-lying spin states, where each orth isomer is generally lower in energy than the corresponding lat isomer. All the MM bonds are assigned formal bond order (fBO) values through MO analysis and/or electron counting. The M₂L₂ orth complexes in many ways mimic the geometrical and electronic structure of known tetragonal lantern complexes. The formal shortness ratio (FSR) and the Wiberg index W_MM for the MM bond correlate well with the fBO values, validating their use as measures of intrinsic MM bond strength.