Understanding the Nonplanarity in Aromatic Metallabenzenes: A σ‑Control Mechanism

Metallabenzenes,
the organometallic counterparts of benzene with one of the C atoms
being replaced by a metal atom, expand the family of aromatics and
further create prospective candidates for novel applications as functional
materials. One intriguing feature of these complexes is that their
MC5 rings do not always constrict into a planar configuration as in
the C6 ring of benzene. Such a deviation has often been attributed
to the unfavorable antibonding interactions between an occupied metal
d orbital and the π orbitals of the C5 moiety. We herein scrutinize
the frontier orbital interactions in both σ and π spaces
in a plethora of metallabenzene complexes using extensive density
functional theory calculations. Unexpectedly, the nonplanarity in
metallabenzenes is found to be hardly related to the π orbitals.
It is the antibonding interaction between an occupied metal d orbital
and the σ orbitals of the C5 moiety that dominates the observed
distortion. Such a σ-control mechanism not only provides an
explanation for the commonly observed nonplanarity in metallabenzenes
but also points out a novel direction toward the rational design of
functional materials with enhanced metalla-aromaticity.