An ab initio analysis of the electronic structure of high-valent, transition-metal alkylidenes as models for olefm metathesis catalysts is presented. The catalyst models studied fall into three categories: "new" metathesis catalyst models-tetrahedral M(0Hh(XH)(CH2) complexes; "old" metathesis catalyst models-tetrahedral MCMY)(CH2) complexes and alkylidene-substituted Mo metathesis catalysts, Mo( OHMNH)(=C(H)Z). The effect on the bonding caused by modification of either the metal, ligands, or alkylidene substituents is considered. For the new models the minimum energy structures result from maximum metal d1rligand p11' bonding and minimum competition among the ligands for the same d11' AO. Rotation about the MC axis increases this competition. The ability of the other multiply bonded ligand (XH) to accommodate the carbene ligand controls the height of the rotation barrier. For Mo and W complexes compression of the M-X-H angle is more facile than that for theRe alkylidene, resulting in a much higher barrier for the latter. A second important result is the greater M+=e- bond polarity in W versus Mo(0Hh(NH)(CH0. Greater polarization correlates with the observed greater metathesis activity for W versus Mo. The bonding in the old models resembles that in the new except for the much greater MC rotational barriers and the lower M+=e- bond polarity in the former. If greater MC bond polarization does indeed lead to greater reactivity, then one would expect that the old metathesis catalysts are less active compared to their new analogues. Finally, the effects of substitution on the alkylidene ligand of a series of Mo alkylidenes was studied. Electron-withdrawing ligands cause the MC bond length to contract; electron donors have the opposite effect.