Systematic Variation of Metal–Metal Bond Order in Metal–Chromium Complexes

In the field of metal–metal bonding, the occurrence of stable, multiple bonds between different transition metals is uncommon, and is largely unknown for different first-row metals. Adding to a recently reported iron–chromium complex, three additional M–Cr complexes have been isolated, where the iron site is systematically replaced with other first-row transition metals (Mn, Co, or Ni), while the chromium site is kept invariant. These complexes have been characterized by X-ray crystallography. The Mn–Cr complex has an ultrashort metal–metal bond distance of 1.82 Å, which is consistent with a quintuple bond. The M–Cr bond distances increases across the period from M = Mn to M = Ni, as the formal bond order decreases from 5 to 1. Theoretical calculations reveal that the M–Cr bonds become increasingly polarized across the period. We propose that these trends arise from increasing differences in the energies and/or contraction of the metals’ d-orbitals (M vs Cr). The cyclic voltammograms of these heterobimetallic complexes show multiple one-electron transfer processes, from two to four redox events depending on the M–Cr pair.