An ab initio investigation into the effects of ligand and substituent modification on the metal-carbon double bond is reported. Prototypical group IVB (Ti, Zr, HO and Group VB (Nb, Ta) alkylidenes are chosen for this study. The MC/LMO/CI (multiconfigurationjlocalized molecular orbital/configuration interaction) procedure is used to examine the electronic structures of these complexes in terms of the prime resonance contributors to the ground-state wave function. The main conclusion drawn from this work is that the intrinsic nature of the metal-carbon double bond can typically be changed only within certain limits by modification of the electronegativity of the ligands (L) and substituents (Z). In other words, the Ta=C bond in H3 TaCH2 is very similar to that in H3 TaCC12 and Cl3 TaCH2 and presumably in experimentally characterized analogues with larger ligands and substituents, e.g., Cp and neopentyl. Significant changes in the electronic structure are effected in three ways: The first way is through the introduction of a highly electropositive substituent, e.g., Li. This makes the metal-carbon bond closer to a triple bond for the Ta-alkylidenes. The second way to change the electronic structure of the alkylidenes significantly is to change the central metal atom. The heaviest members of groups IVB (HO and VB (Ta) are the most nucleophilic at the a-carbon. The third way in which the metal-carbon bond could be significantly altered is through the use of 1r-donor substituents. The introduction of 1r-donor substituents on Ca increases the electrophilicity of the a-carbon.