Representation of Three-Center-Two-Electron Bonds in Covalent Molecules with Bridging Hydrogen Atoms

Many compounds can be represented well in terms of the two-center-two-electron (2c-2e) bond model. However, it is well-known that this approach has limitations; for example, certain compounds require the use of three-center-two-electron (3c-2e) bonds to provide an adequate description of the bonding. Although a classic example of a compound that features a 3c-2e bond is provided by diborane, B2H6, 3c-2e interactions also feature prominently in transition metal chemistry, as exemplified by bridging hydride compounds, agostic compounds, dihydrogen complexes, and hydrocarbon and silane a-complexes. In addition to being able to identify the different types of bonds (2c-2e and 3c-2e) present in a molecule, it is essential to be able to utilize these models to evaluate the chemical reasonableness of a molecule by applying the octet and 18-electron rules; however, to do so requires determination of the electron counts of atoms in molecules. Although this is easily achieved for molecules that possess only 2c-2e bonds, the situation is more complex for those that possess 3c-2e bonds. Therefore, this article describes a convenient approach for representing 3c-2e interactions in a manner that facilitates the electron counting procedure for such compounds. In particular, specific attention is devoted to the use of the half-arrow formalism to represent 3c-2e interactions in compounds with bridging hydrogen atoms.