Trends in the Bond Multiplicity of Cr₂, Cr₃, and Cr₂M (M = Zn, Ni, Fe, Mn) Complexes Extracted from Multiconfigurational Wave Functions

Extended metal atom chains constitute an interesting class of molecules from both theoretical and applied points of view. In the chromium-based series Cr2M­(dpa)4X2 (with M = Zn, Ni, Fe, Mn, Cr), the direct metal–metal interactions span a wide range of possibilities and so do their associated properties. The multiplicity and symmetry components of the metal–metal bond are herein analyzed via the effective bond order (EBO) concept using complete active space self-consistent field wave functions and compared with similar bimetallic Cr2L4X2 systems. The bond multiplicity follows a trend dominated by the Cr–Cr distance which, in turn, depends on the nature of the axial ligand (X). Cr2M compounds present asymmetric structures with virtually no interaction between the Cr2 unit and M, whereas fully symmetric structures with delocalized bonding among the three metals are also possible in Cr3 complexes. In such cases, a strategy that involves localization of the molecular orbitals into each Cr–Cr pair is applied to quantify the contribution of each pair to the overall metal–metal bond multiplicity