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.