Small Molecule Elimination from Group IVB (Ti, Zr, Hf) Amido Complexes
Date: May 1993
Creator: Cundari, Thomas R., 1964- & Gordon, Mark S.
Description: This article discusses small molecule elimination from group IVB (Ti, Zr, Hf) amido complexes. An ab initio quantum chemical analysis of HX (X = H, CH₃, Cl, NH₂, SiH₃) elimination by group IVB (Ti, Zr, Hf) amidos (H₂(X)M - NH₂ → H₂M = NH + HX), of interest in the context of CVD precursor design, is reported. Several deductions may be drawn from the calculations. First, in the transition state (TS) for HX elimination, electropositive and electroneutral X give rise to metal-transannular hydrogen (Ht) distances only slightly longer than normal metal-terminal hydride bonds lengths, while electronegative X groups yield substantially longer MHt distances. Second, the HX elimination barrier (∆Hǂelim) is lower when HX is polarized Hδ- • Hδ+ (X = SiH₃) or nonpolar (X = H). Third, a plot of calculated ∆Hǂelim versus MHt distances in the TS. Fourth, analysis of the electronic structure along the intrinsic reaction coordinate (IRC) supports the importance of N-H•••M agostic interactions preceding N-H scission. Fifth, the IRC shows the MHt distance decreasing as Ht is transferred from N to X, reaching a minimum when the transfer is roughly half complete, and then increasing once more is HX is eliminated. These results point to the ...
Contributing Partner: UNT College of Arts and Sciences
Permallink:digital.library.unt.edu/ark:/67531/metadc107776/
Principal Resonance Contributors to High-Valent, Transition-Metal Alkylidene Complexes
Date: July 1991
Creator: Cundari, Thomas R., 1964- & Gordon, Mark S.
Description: This article discusses principal resonance contributors to high-valent, transition-metal alkylidene complexes. The results of ab initio calculations are reported for prototypical high-valent, alkylidene complexes. Stationary points on each potential energy surface are characterized and compared to experimental information where available; as long as a suitably flexible valence basis set is used, good agreement between theoretically calculated and experimentally determined geometries is obtained. The complexes of interest include group IVB (Ti, Zr and Hf) and group VB (Nb and Ta) alkylidenes with hydride ligands as well as models for the four-coordinate, olefin metathesis catalysts (Mo-, W-, and Re-alkylidenes) which have been recently synthesized and characterized. In light of the fact that much of the discussion concerning the reactivity of transition-metal carbene complexes has been presented in terms of the resonance contributors derived from rearranging the electrons in the M-C σ and π orbitals, the minima obtained from the portion of the study are then subjected to a further procedure to calculate these contributions. Resonance structures in which the carbon is the negative end of the M-C bond (i.e., nucleophilic resonance structures) contribute 50% to the ground-state wave function of these complexes. Those in which the carbon is formally neutral account for ...
Contributing Partner: UNT College of Arts and Sciences
Permallink:digital.library.unt.edu/ark:/67531/metadc107773/
High-Valent Transition-Metal Alkylidene Complexes: Effect of Ligand and Substituent Modification
Date: 1992
Creator: Cundari, Thomas R., 1964- & Gordon, Mark S.
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 ...
Contributing Partner: UNT College of Arts and Sciences
Permallink:digital.library.unt.edu/ark:/67531/metadc107774/