Structural Increments for 11-Vertex <i>n</i><i>ido-</i>Phospha- and Aza(carba)boranes and -borates; Dependence of Energy Penalties on the Extent of Electron Localization

Relevant structural features and corresponding energy penalties were determined that allow to easily estimate the relative stabilities of 11-vertex <i>nido</i>-phospha- and aza-substituted boranes, borates, carbaboranes, and carbaborates. For this purpose, density functional theory computations at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d)+ZPE level were carried out to determine the relative energies of 95 phospha- and 46 aza(carba)boranes and -borates. Energy penalties assigned to disfavoring structural features show additive behavior and excellent precision with respect to the computed results, as in the case of 6- and 11-vertex <i>nido-</i>carboranes and -borates. An unsubstituted phosphorus atom was found to possess energy penalties quite similar to those of the three-electron-donating H−C group. A bare nitrogen atom has energy penalties much larger than those of a bare phosphorus atom. Four-electron-donating RP and RN moieties, however, have even more adverse energy penalties. The disfavoring effects of heteroatoms in a borane cluster are determined by the amount of electron localization, that is, primarily by the number of skeletal electrons that formally originate from the heterogroup and secondarily by the electronegativity. Heteroatom energy penalties are independent of the type of the other heteroatoms present in the same cluster. Some novel phospha(carba)borane geometries with bare and exo-substituted phosphorus atoms in the same cluster have favorable thermodynamic stabilities competitive with those of known isomers.