Three-Centre Two-Electron Bonds and the Absence of an Fe–Fe bond in Fe2(CO)9

An article that describes a procedure for counting electrons in compounds that feature μ–L groups was recently reported in Chemical Communications. Significantly, this article reconciles the long standing problem between the M–M bond orders calculated theoretically for certain bridging carbonyl compounds, e.g. Fe2(CO)9, and those predicted by using electron counting. See: “The Occurrence and Representation of Three-Centre Two-Electron Bonds in Covalent Inorganic Compounds” by Jennifer C. Green, Malcolm L. H. Green, and Gerard Parkin Chem. Commun. 2012, 48, 11481–11503.