Description: This article offers comments on the competitive preferential solvation theory. Abstract: Simple additive relationships for the physico-chemical properties of a solute dissolved in binary solvent mixtures are developed from the competitive preferential solvation model. Additive expressions for solute mole fraction solubility and logarithm of solute solubility are mathematically identical to equations derived previously from the microscopic partition and basic nearly ideal binary solvent models. Calculated values based on the various additive relationships are compared to carbazole solubilities in ten binary solvent mixtures containing dibutyl ether with n-hexane, n-heptane, n-octane, cyclohexane, cyclo-octane, methylcyclohexane, iso-octane, n-hexadecane, squalane and t-butylcyclohexane.

Description: This article discusses molecular modeling of vanadium-oxo complexes. Abstract: A force field for vanadium-oxos was developed and tested with a variety of complexes with coordination numbers of 5 or 6 and formal oxidations states of +4 or +5 on the metal. Similarly, a semiempirical quantum mechanical method for transition metals was extended to vanadium. In this research soft and hard ligands were studied, as were ligands coordinated through single, multiple, and dative bonds. Despite the diversity of vanadium coordination chemistry, generally good modeling is achieved in a fraction of the time with less computational resources using molecular mechanics and semiempirical quantum mechanics. The L₄V⁴⁺O and L₅V⁵⁺O groups were emphasized given their prevalence and importance. In general, the predictive ability was superior for the former structural motif. The combination of molecular mechanics and semiempirical quantum calculations provide an effective and efficient tool for analysis of the steric and electronic energy differences between isomers.

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 ...

Description: This article discusses structural dichotomy in six-coordinate d⁰ complexes. Utilization of the bulky acetylide, ᵗBu₃SiC≡C⁻, enabled the synthesis of several early metal polyacetylides. Addition of NaC≡CH to ᵗBu₃SiBr in dimethyl sulfoxide afforded ᵗBu₃SiC≡CH, which was deprotonated to yield ᵗBu₃SiC≡CLi. Treatment of ZrCl₄, HfCl₄, adn TaCl₅ with varying amounts of ᵗBu₃SiC≡CLi gave {(THF)₂Li(ᵗBu₃SiC≡C)₂}Zr(C≡CSiᵗBu₃)₃(THF) (1; THF = tetrahydrofuran), {Et₂O)Li(ᵗBu₃SiC≡C)₂}Hf(C≡CSiᵗBu₃)₃(OEt₂) (2), {Li(ᵗBu₃SiC≡C)₃}₂M(M = Zr, 6; Hf, 7), and {Li(ᵗBu₃SiC≡C)₃}Ta(C≡CSiᵗBu₃)₃ (3). Metathesis of 3 with KOTf generated KTa(C≡CSiᵗBu₃)₆ (4) and cation sequestration of 4 with crypt 2.2.2 provided [K(crypt 2.2.2)][Ta(C≡CSiᵗBu₃)₆](5). Single-crystal X-ray structural studies determined the structures (core symmetry) of 1 (Oh), 2, (Oh), 3 (D₃), 5 (D₃), 6 (Oh), and 7 (Oh). The D₃h to D₃ twist in 3 and 5 has a steric origin, and the counterion position appears inconsequential. Origins of the structural preferences illustrated by the dichotomous twisted trigonal prismatic and octahedral cores of the d⁰ hexaacetylides 5 and 6 were probed through density functional (ADF) and effective core potential (GAMESS) calculations. The structural difference results from a lessening electronic preference for the trigonal prism-primarily a greater HOMO/LUMO gap-upon moving from Ta to Zr, minor steric factors, and increased interligand repulsions in the dianion (VSEPR).

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 ...