Iron enters the pentagon: Quantum chemical calculations using gradient‐corrected DFT predict that the cations Fe(Sb5)+ and Fe(Bi5)+ in the electronic singlet state have planar (D5h) equilibrium ...geometries (see scheme). Analysis of the electronic structure shows that the molecules are metal‐centered six‐π‐electron aromatic species with strong iron–ligand π bonds which involve the d(π) atomic orbitals of the Fe center and the degenerate π orbital of the ring. The calculated 57Fe NMR chemical shifts indicate extremely high deshielding of the metal nucleus.
Computational exploration of carboranes of formula CnBmq+ reveals that more than one hypercoordinated carbon atom can be enclosed by a peripheral ring comprising a suitable number of boron atoms. The ...C2B8, C3B93+, and C5B11+ species (the LUMO of the latter is shown) are stabilized by substantial Hückel π aromaticity. Furthermore, multicenter σ bonding helps bind the inner carbon units to the boron perimeters, though they can freely rotate relative to one another.
The equilibrium geometries of Me2XCl2 for X = C, Si, Ge, Sn, Pb, Ti, Zr, and Hf are calculated at the HF and MP2 levels of theory using valence basis sets of DZ+P quality. The calculated geometries ...are in good agreement with experimental gas-phase values. The Cl−X−Cl angle is always smaller than the C−X−C angle when X is a main group element C−Pb. This is in agreement with Bent's rule. The opposite relationship is predicted for the transition metal compounds. The calculated Cl−X−Cl angle is significantly larger than the C−X−C angle for X = Ti, Zr, and Hf. The different order of the Cl−X−Cl and C−X−C angles between the main group and the transition metal compounds is explained by the energy levels of the valence orbitals of the central atom X. The transition metals have mainly sd x -hybridized bonds, while the main group elements have sp x -hybridized bonds. The valence s orbital of the main group elements is always below the p valence orbitals, but the valence s orbital of the transition metals is above the valence d orbitals. The energetically lower lying valence orbital concentrates in bonds toward the more electropositive methyl substituents yielding bond angles C−X−C > Cl−X−Cl when X is a main group element and C−X−C < Cl−X−Cl when X is a transition metal. It is suggested that Bent's rule should be formulated in a more general way: “The energetically lower lying valence orbital concentrates in bonds directed toward electropositive substituents”.
Transition metal catalysis belongs to the most important chemical research areas because a ubiquitous number of chemical reactions are catalyzed by transition metal compounds. Many efforts are being ...made by industry and academia to find new and more efficient catalysts for chemical processes. Transition metals play a prominent role in catalytic research because they have been proven to show an enormous diversity in lowering the activation barrier for chemical reactions. For many years, the search for new catalysts was carried out by trial and error, which was costly and time consuming. The understanding of the mechanism of the catalytic process is often not very advanced because it is difficult to study the elementary steps of the catalysis with experimental techniques. The development of modern quantum chemical methods for calculating possible intermediates and transition states was a breakthrough in gaining an understanding of the reaction pathways of transition metal catalyzed reactions. This volume, organized into eight chapters written by leading scientists in the field, illustrates the progress made during the last decade.; The reader will obtain a deep insight into the present state of quantum chemical research in transition metal catalysis.
The syntheses of the phosphane complexes M(CO)5PH3 (M = Mo, W), W(CO)5PD3, and W(CO)5PF3 and the results of X-ray structure analyses of W(CO)5PH3 and Mo(CO)5PCl3 are reported. Quantum-chemical DFT ...calculations of the geometries and M−P bond dissociation energies of M(CO)5PX3 (M = Cr, Mo, W; X = H, Me, F, Cl) have been carried out. There is no correlation between the bond lengths and bond dissociation energies of the M−P bonds. The PMe3 ligand forms the strongest and the longest M−P bonds of the phosphane ligands. The analysis of M−PX3 bonds shows that PCl3 is a poorer σ donor and a stronger π(P) acceptor than the other phosphanes. The energy decomposition analysis indicates that the M−P bonds of the PH3 and PMe3 complexes have a higher electrostatic than covalent character. The electrostatic contribution is between 56 and 66% of the total attractive interactions. The orbital interactions in the M−PH3 and M−PMe3 bonds have more σ character (65−75%) than π character (25−35%). The M−P bonds of the halophosphane complexes M(CO)5PF3 and M(CO)5PCl3 are nearly half covalent and half electrostatic. The π bonding contributes ∼50% to the total orbital interaction.
Quantum chemical calculations using gradient-corrected (B3LYP) density functional theory have been carried out to investigate the mechanism of the oxidative cleavage of alkenes by ruthenium ...tetraoxide. The initial reaction of the tetraoxide with the olefin occurs via a 3+2 cycloaddition as in the case of osmium tetraoxide. The results clearly show that the bond cleavage does not take place at the primary adduct, but much later in the reaction path. After the formation of the ruthenium(VI)dioxo-2,5-dioxolane, the reaction proceeds with the addition of a second olefin to yield ruthenium(IV)-bis(2,5-dioxolane), which in turn becomes oxidized first to rutheniumoxo(VI)-bis(2,5-dioxolane) 6(Ru) and then to ruthenium(VIII)-dioxo-bis(2,5-dioxolane) 7(Ru). Only in complexes containing the metal center in the formal oxidation state +VIII are low activation barriers for C−C bond cleavage and exothermic formation of carbonyl compounds as products calculated. The lowest activation barrier, ΔH ⧧ = 2.5 kcal/mol, is calculated for the C−C bond breaking reaction of 7(Ru) which is predicted as the pivotal intermediate of the oxidation reaction. The calculations of the oxidation reaction with OsO4 show that those reactions where the oxidation state of the metal increases have larger activation barriers for M = Ru than for M = Os, while reactions which reduce the oxidation state have a lower activation barrier for ruthenium compounds. Also, reactions which increase the oxidation state of the metal are in the case of M = Os more exothermic than for M = Ru. In this work, all important points of the potential energy surface (PES) are reported, and the complete catalytic cycle for the oxidative cleavage of olefins by ruthenium tetraoxide is presented.
Another noble gas conquered Frenking, G
Nature (London),
08/2000, Letnik:
406, Številka:
6798
Journal Article
Recenzirano
Khriachtchev et al report in a study the synthesis of the first compound containing the noble gas argon. This leaves only two stable elements in the periodic table--helium and neon--for which no ...neutral compound exists.
TiN bulk and surface energy and hydrogen atom adsorption at three different sites have been studied using density functional theory (DFT) with local and non‐local exchange–correlation functionals. ...Calculations of surface energies confirm the experimental findings that the (100) surface has the lowest and the (111) surface the highest surface energy, respectively. Adsorption of H on top of Ti atom is more favorable by 1.7 kcal/mol than on top of N atom and is in agreement with plane‐wave calculations and experimental results available in the literature. We also discuss the surface diffusion scenario of H on the (100) surface of TiN.
