Ab initio (MP2/6-311+G*) and density functional theory (B3LYP/6-311+G*) calculations have been performed to determine the bonding nature of N, P and As (pnicogen) with B or C in ylides and their boron analogs. The compounds studied refer to the formulas R₃XCR'₂ and R₃XBR' (X=N, P, As; R=H, F; and R'=H, SiH₃). The computed electron density has been analyzed by means of the Atoms in Molecules (AIM) theory and the Electron Localization Function (ELF) methods. In addition, the rotational barriers were calculate for the X-C and X-B bonds at the CASMP2/6-311+G* level to elucidate the multiple bonding character for these bonds. The geometrical and electronic results indicated that the N ylides differ remarkably from the remaining pnicogen (P, As) ylides, the former yielding clear single bonds, while the latter showed stronger multiple bonds. Moreover, the fluorine substituents strengthened the X-C and X-B bonds, reducing the bond distance, increasing the electron density and augmenting the planarity at the C and B atoms. However, the SiH3 groups affected only the planarity at the C atoms for the organic ylides. This indicates how the electronegativity of the different substituents influence the central X-C and X-B bonds: if the substituent pulls charge from the bond in the direction towards the pnicogen, the bond is reinforced and it is more likely to present double-bond characteristics. This is not accomplished by substituents pushing charge in the aforementioned direction. Differences between the organic ylides and their boron derivatives have been found. Boron analoges presented a remarkable asymmetric X-B bonds, with a rotation barrier of ca. 30 kcal/mol, caused by a strong repulsion between the lone pairs of the XH₃ unit and that of boron.

ELF representation at 0.7 for rotamer 5a|, at the B3LYP/6-311+G*//B3LYP/6-311+G* level.