Are carbon—halogen double and triple bonds possible?

Abstract

The carbon—halogen and carbon—chalcogen bonding of 84 molecules was investigated utilizing local vibrational modes calculated at the M06‐2X/cc‐pVTZ level of theory including anharmonicity corrections in all cases. The relative bond strength order of each CX or CE bond (X = F, Cl; E = O, S) was derived from the local CX or CE stretching force constant and compared with trends of calculated bond lengths r and bond dissociation energies (BDE) obtained at the G3 level of theory. It is shown that both bond length r and BDE are not reliable bond strength descriptors. The CX double bond is realized for some Cl‐substituted carbenium ions, however, not for the corresponding F‐derivatives. Diatomic CF+ and CCl+ possess fully developed double bonds but not, as suggested in the literature, triple bonds. Halonium ions have fractional (electron‐deficient) CX bonds, which can be stabilized by σ‐donor substituents or by an increased polarizability of the halogen atom as with Cl. Bridged halonium ions are more stable than their acyclic counterparts, which results from more effective two‐electron‐three‐center bonding. © 2014 Wiley Periodicals, Inc.