The effect of substituent has been theoretically explored in the case of polarized phosphaalkenes formed with an aminocarbene and a phosphinidene. The shape of the experimentally characterized structures has been explained in terms of electronic and steric effects. The balance between sigma-donation and pi-back-donation as a function of the substituent has been explored by the use of electron localization function (ELF) analysis and correlated with the bond dissociation energy and geometrical structure. The aromaticity of the carbene ring, obtained by NICS calculation, has been achieved.