The electronic and molecular structures of the complexes [(eta(5)-C5H5)(CO)(2)M EMe] and [(eta(5)-C5H5)(CO)(3)M-EMe] (M = Cr, Mo, W; E = Si, Sn, Pb) are calculated at the density-functional theory (DFT) level using the exchange correlation functionals B3LYP and BP86. The theoretically predicted bond lengths and angles of the model compounds are in excellent agreement with experimental values. The calculations reveal the presence of a strong M E triple (sigma + 2 pi) bond in [(eta(5)-C5H5)(CO)(2)M EMe]. The M-E bond lengths in [(eta(5)-C5H5)(CO)(3)M-EMe) are longer than those expected for a single bond. The nature of the M EMe and M-EMe interactions was analyzed with charge and energy decomposition methods. In the M EMe bond, the M-E sigma-bonding orbitals are always polarized toward the silicon, tin, and lead atoms, and the polarization increases from chromium to tungsten. In contrast, in the M-EMe bond, the M-E sigma-bonding orbitals are significantly polarized toward the metal atom. The hybridization at the metal atoms in the M E bonds has d character in the range 60.6-68.8%, while in the M-E bonds has large d character which is always >86% of the total atomic orbital contribution. In the complexes [(eta(5)-C5H5)(CO)(2)M EMe], the contributions of the electrostatic interactions, Delta E-elstat, and the covalent bonding, Delta E-orb, have nearly the same values for silylidyne and germylidyne complexes, while for the stannylidyne and plumbylidyne complexes, the electrostatic interactions, Delta E-elstat, are greater than the orbital interaction, Delta E-orb. The covalent bonding has a high degree of pi-character. The total interaction energy Delta E-int in the compound [(eta(5)-C5H5)(CO)(3)M-EMe] is less attractive than those in the complexes [(eta(5)-C5H5)(CO)(2)M EMe]. The M-ER bonds have a slightly lower degree of covalent bonding (34.9-44.9%) than the M EMe bonds (42.1-50.2%). The drastic difference between the two classes of compounds are found for the degree of a" (pi) bonding. The contribution of Delta E-pi to the covalent term Delta E-orb is much higher in the M EMe bonding (41.6-42.6%) than in the M-EMe bonding (17.1-20.4%). While the pi bonding contribution in [(eta(5)-C5H5)(CO)(3)M-EMe] are weaker than those in [(eta(5)-C5H5)(CO)(2)M EMe], the sigma-bonding contribution in the former compounds are stronger than those in the latter.