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.