By application of empirical and semi-empirical methods the enthalpies of formation for series of XN=C=Y molecules (X = F, Cl, Br, I; Y = O, S, Se) were calculated. Empirical procedures was used for calculation of enthalpies of formation for X=C=C=X molecules with X = O, S, Se. The possibility of synthesis of carbon monoxide dimer O=C=C=O is discussed.