Description: This article discusses high-valent transition-metal alkylidene complexes. Abstract: 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, Hf) and Group VB (Nb, Ta) alkylidenes are chosen for this study. The MC/LMO/CI (multiconfiguration/localized 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 H₃TaCCl₂ and Cl₃TaCH₂ 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 (Hf) and ...

Description: This articles discusses transition metal imido complexes. A wide range of transition metal imido (TMI) complexes is studied using ab initio molecular orbital (MO) calculations. The main computational point of interest is the further testing of effective core potentials (ECPs) and valence basis sets to allow for the accurate calculation of properties for reasonably sized transition metal complexes. On the chemical side, several results from the study are to be noted. The agreement between geometries for calculated models and their experimental counterparts ranges from very good to excellent, as found in previous studies of multiply bonded transition metal-group IVA complexes. Taken together, these data suggest that the accuracy one has come to expect for the prediction of structural properties for main-group compounds may yet become a reality for transition metal complexes. The correct prediction of molecular structure also leads one to infer that the bonding in these complexes is also accurately described. The MC/LMO/CI (multiconfigurational/localized MO/configuration interaction) technique shows that eight resonance structures are most significant in the description of the metal-imido linkage. Three of these account for roughly two-thirds of the total contributions; two resonance structures, both possessing a dative σ bond, correspond to novel bonding descriptions for TMI ...