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

In this study the nature of the bonding in a series of dimethylhalonium ylides (fluoronium, chloronium, bromonium and iodonium) was analyzed by means of topological methodologies (AIM and ELF analysis), to document the changes in the nature of the C−X bonds (X = F, Cl, Br, I) upon the series. For the sake of comparison the same study was performed on the corresponding dimethylhalonium cations (XC 2H 6+) and the XCH 3series. The wave functions used for the topological analysis were obtained at B3LYP level using extended triple-ζ basis sets. The formation of the cationic XC 2H 6+structures can be interpreted to arise from the interaction between the XCH 3and CH 3+moieties. The resultant structures can be explained in terms of the superposition of two electrostatically interacting and two dative mesomeric structures. The halogen−carbon bonds have all the characteristics of the charge-shift (CS) bonds. The analysis of the C−X bond in the XC 2H 5series shows a progressive reinforcing of the CH 3X−CH 2bond, from FC 2H 5that can be considered as formed from two fragments, FCH 3and CH 2, to IC 2H 5, in which the CH 3I−CH 2bond has all the features of a multiple bond involving atoms bearing lone pairs. Particularly interesting is BrC 2H 5, in which a special type of bond (hybrid covalent-dative double bond) has been characterized. The energetic stability of the XC 2H 5structures with respect to the dissociation into the XCH 2+ CH 3and XCH 3+ CH 2ground-state fragments was studied in detail. [ABSTRACT FROM AUTHOR]