Metal–metal bond distances and bond orders in dimanganese complexes with bidentate ligands: scope for some very short Mn–Mn bonds

Abstract

Binuclear manganese complexes with covalent Mn–Mn bonds include dimanganese carbonyl complexes and dimanganese non-carbonyl complexes with bidentate anionic ligands. Density functional theory (DFT) using the M06-L functional is used to study some known dimanganese complexes and three series of model paddlewheel-type dimanganese complexes M₂L_x and M₂L_x·2H₂O (L = HNCHNH, HNC(NH₂)NH and OCHO respectively; x = 2, 3, 4). The DFT results for model structures for known dimanganese complexes agree well with the experimental structures and lead to predictions of ground state spin multiplicities. For the model paddlewheel-type complexes, predictions are made with regard to ground state spin multiplicities, and Mn–Mn bond lengths. Formal bond orders (fBO) from 0.5 to 4.5 for all the Mn–Mn bonds are assigned by electron counting. Molecular orbital analysis yields Mn–Mn bond orders (BO) matching the fBO values. Four “super-short” Mn–Mn bonds (lengths 1.618 to 1.639 Å) are found in higher energy isomers, along with other short Mn–Mn quadruple bonds. The effects of axial ligation by two H₂O ligands are also noted. The Mn–Mn bond lengths are categorized into distinct ranges as per the Mn–Mn fBO values from 0.5 to 4.5, ranges which are compared with other experimental and computational estimates.