The molecular and electronic structures and bonding analysis of terminal cationic metal-ylyne complexes (MeCN)(PMe3)(4)M equivalent to EMes](+) (M = Mo, W; E = Si, Ge, Sn, Pb.) were investigated using DFT/BP86/TZ2P/ZORA level of theory. The calculated geometrical parameters for the model complexes are in good agreement with the reported experimental values. The M-E sigma-bonding orbitals are slightly polarized toward E except in the complex [(MeCN)(PMe3)(4)W(SnMes)](+), where the M-E sigma-bonding orbital is slightly polarized toward the W atom. The M-E pi-bonding orbitals are highly polarized toward the metal atom. In all complexes, the pi-bonding contribution to the total M equivalent to EMes bond is greater than that of the sigma-bonding contribution and increases upon going from M = Mo to W. The values of orbital interaction Delta E-orb are significantly larger in all studied complexes I-VIII than the electrostatic interaction AE(elstat). The absolute values of the interaction energy, as well as the bond dissociation energy, decrease in the order Si > Ge > Sn > Pb, and the tungsten complexes have stronger bonding than the molybdenum complexes.