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 3 series. The wave functions used for the topological analysis were obtained at B3LYP level using extended triple-zeta basis sets. The formation of the cationic XC 2H 6 (+) structures can be interpreted to arise from the interaction between the XCH 3 and 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 5 series shows a progressive reinforcing of the CH 3X-CH 2 bond, from FC 2H 5 that can be considered as formed from two fragments, FCH 3 and CH 2, to IC 2H 5, in which the CH 3I-CH 2 bond 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 5 structures with respect to the dissociation into the XCH 2 + CH 3 and XCH 3 + CH 2 ground-state fragments was studied in detail.