Topological Insights into the Nature of the Halogen−Carbon Bonds in Dimethylhalonium Ylides and Their Cations

Charge-Shift Bonding: A New and Unique Form of Bonding

Charge-shift bonds (CSBs) constitute a new class of bonds different than covalent/polar-covalent and ionic bonds. Bonding in CSBs does not arise from either the covalent or the ionic structures of the bond, but rather from the resonance interaction between the structures. This Essay describes the reasons why the CSB family was overlooked by valence-bond pioneers and then demonstrates that the unique status of CSBs is not theory-dependent. Thus, valence bond (VB), molecular orbital (MO), and energy decomposition analysis (EDA), as well as a variety of electron density theories all show the distinction of CSBs vis-à-vis covalent and ionic bonds. Furthermore, the covalent-ionic resonance energy can be quantified from experiment, and hence has the same essential status as resonance energies of organic molecules, e.g., benzene. The Essay ends by arguing that CSBs are a distinct family of bonding, with a potential to bring about a Renaissance in the mental map of the chemical bond, and to contribute to productive chemical diversity.

The salts of chloronium ions R-Cl(+)-R (R = CH3 or CH2Cl): formation, thermal stability, and interaction with chloromethanes

The interaction of CH3Cl/CD3Cl or CH2Cl2/CD2Cl2 with the carborane acid H(CHB11Cl11) (abbreviated as H{Cl11}) generates the salts of CH3-{Cl11} and CH2Cl-{Cl11} and their deuterio analogs, respectively, which are analogs of the salts of asymmetric chloronium cations. Next, salts of chloronium cations CH3-Cl(+)-CH3, ClCH2-Cl(+)-CH2Cl, and ClCH2-Cl(+)-CH3 and their deuterio analogs were obtained from the above compounds. The asymmetric ClCH2-Cl(+)-CH3 cation was found to be unstable, and at ambient temperature, slowly disproportionated into symmetric cations (CH3)2Cl(+) and (CH2Cl)2Cl(+). At a high temperature (150 °C), disproportionation was completed within 5 minutes, and the resulting cations further decomposed into CH3-{Cl11} and CH2Cl-{Cl11}. The molecular fragment ClCH2-(X) of the compounds (X = {Cl11}, -Cl(+)-CH2Cl, or -Cl(+)-CH3) is involved in exchange reactions with CH2Cl2 and CHCl3, converting into CH3-(X) with the formation of chloroform and CCl4, respectively.

Quantitative assessment of the multiplicity of carbon-halogen bonds: carbenium and halonium ions with F, Cl, Br, and I

CX (X = F, Cl, Br, I) and CE bonding (E = O, S, Se, Te) was investigated for a test set of 168 molecules using the local CX and CE stretching force constants k(a) calculated at the M06-2X/cc-pVTZ level of theory. The stretching force constants were used to derive a relative bond strength order (RBSO) parameter n. As alternative bond strength descriptors, bond dissociation energies (BDE) were calculated at the G3 level or at the two-component NESC (normalized elimination of the small component)/CCSD(T) level of theory for molecules with X = Br, I or E = Se, Te. RBSO values reveal that both bond lengths and BDE values are less useful when a quantification of the bond strength is needed. CX double bonds can be realized for Br- or I-substituted carbenium ions where as suitable reference the double bond of the corresponding formaldehyde homologue is used. A triple bond cannot be realized in this way as the diatomic CX(+) ions with a limited π-donor capacity for X are just double-bonded. The stability of halonium ions increases with the atomic number of X, which is reflected by a strengthening of the fractional (electron-deficient) CX bonds. An additional stability increase of up to 25 kcal/mol (X = I) is obtained when the X(+) ion can form a bridged halonium ion with ethene such that a more efficient 2-electron-3-center bonding situation is created.