Description: This article discusses the calculation of a methane C-H oxidative addition trajectory. Abstract: An effective core potential (ECP), parallel supercomputing study of methane activation by 14-electron, Ir(PH₃)₂(X) complexes (X = H, Cl) is presented. Considerable weakening of the coordinated methane C-H bond occurs upon formation of an ɳ²-CH coordinated (X)(PH₃)₂Ir•••HCH₃ adduct. A more strongly bound adduct (with greater weakening of the coordinated C-H bond) occurs when X = Cl versus X = H. The calculated Ir(PH₃)₂(H) + CH₄ → Ir(PH₃)₂(H)₂(Me) reaction enthalpy is -12.8 kcal mol⁻¹, and -41.6 kcal mol⁻¹ for the chloro analogue. The intrinsic reaction coordinate is calculated and compared to an experimental trajectory. Analysis of the wave function along the intrinsic reaction coordinate (IRC) suggests that although donation of electron density from methane to metal is essential for adduct formation, it is not until backdonation to σ* сʜ increases that the C-H bond is activated and cleaved. The electronic and molecular structure of the reacting system along the IRC suggest a two-stage mechanism: substrate to complex donation is important in the early part of the reaction (electrophilic stage) while complex to substrate backdonation is necessary later on (nucleophilic stage) for C-H scission. Finally, comparison of IRCs for ...

Description: This article discusses an effective core potential study of transition-metal chalcogenides. Abstract: A structural analysis is reported of roughly 150 transition-metal (TM)-chalcogenido complexes in a variety of chemical environments. With few exceptions, agreement between calculated and experimental geometries is excellent. The research provides convincing evidence that computational methods employed are adequately describing the bonding in these diverse TM complexes. Interesting trends in relative TMCh (Rмсh-Rмсh) bond lengths are found. Experimental and computational data show that other than the zirconocene-and halfnocene-oxos there is similar behavior in relative bond lengths for widely varying TM-chalcogenido complexes. Relative bond lengths versus oxo (S-O, Se-O, and Te-O) in group IVB metallocenes tend to be larger than for other families of complexes and show less variation among the heavier chalcogens (Se-S, Te-S, and Te-Se). Analysis of localized wave functions for Cp₂ZrCh point to a greater contribution from a singly-bonded Zr-Ch structure (relative to Zr=Ch) when Ch is O compared to heavier chalcogens. Taken together, the data suggest that there is a fundamental difference in the Zr-oxo (and Hf-oxo) bond in relation to heavier chalcogens, consistent with recent experimental data. In previous studies of multiply bonded TM complexes the authors have focused on the ability of ECPs ...

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 article discusses modeling nonlinear optical properties of transition metal complexes. Nonlinear optical (NLO) properties of transition metal complexes are studied using quantum chemical calculations. By comparison with all electron calculations, effective core potentials have been shown to be competent for the calculation of NLO properties as long as the valence basis sets are comparable. While overall the basis set effects are important for calculation of NLO properties, they are found to be less important for the central transition metal than for the surrounding ligands. Augmenting the basis set of main group elements with diffuse, s, p, and d functions in a proper way could provide the best compromise between speed and accuracy of the computation. Interesting trends are found in the calculation of NLO properties of [MO₄]q⁻. Both polarizability (α) and second hyperpolarizability (y) decrease toward the right across the transition series. The second series [MO₄]q⁻ have the largest α among the three metalates in a triad. For group IVB and VB complexes with larger charges (-4 and -3, respectively), the second series [MO₄]q⁻ have the largest y, while for groups VIB, VIIB, and VIII, with less anionic metalates (-2, -1, and 0, respectively), the third series metalates have ...

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 ...