The electronic and molecular structures of the title compounds have been investigated using density functional theory within the local density approximation, adding nonlocal corrections to exchange and correlation energy as a perturbation. The transition metal main group double bond was analyzed in terms of sigma and pi bond contributions. The main difference between chromium pentacarbonyl complexes with the carbene fragment compared to their higher homologues is a significant drop in the intrinsic pi bond strength : D-pi,D-int(M=C) = 202 kJ/mol, D-pi,D-int (M=Si) = 82 kJ/mol, D-pi,D-int (M=Ge) = 72 kJ/mol, and D-pi,D-int (M=Sn) = 51 kJ/mol. For the carbene complexes with transition metals of the chromium triad, the intrinsic pi bond strengths are very similar : D-pi,D-int (Cr=C) = 202 kJ/mol, D-pi,D-int (Cr=Mo) = 204 kJ/mol, and D-pi,D-int (Cr=W) = 221 kJ/mol. Relativistic effects are responsible for the increased pi bond strength in the tungsten complex. The bond strengths for the metal carbon double bond rank as BE(W=C) > BE(Cr=C) > BE(Mo=C). Molecular orbital arguments are provided to explain the calculated trends. Further, the question of the rotational barriers around the M=E bond is addressed, and the differences in the geometry for the eclipsed as well as for the staggered conformation of (CO)(5)M=EH(2) are analyzed.