Matthias Lein

Towards a rigorously defined quantum chemical analysis of the chemical bond in donor /acceptor complexes

Abstract The results of an energy decomposition analysis of various classes of donor–acceptor complexes of transition metals and main-group elements are discussed. It is shown that the nature of the chemical bond can be quantitatively identified in terms of Pauli repulsion, electrostatic attraction and covalent bonding. The covalent and electrostatic contributions to the interatomic attraction can be precisely given by using a well defined partitioning method in conjunction with accurate quantum chemical calculations of the geometries and bond energies. This is shown for six classes of donor–acceptor complexes: (a) transition metal carbonyl complexes; (b) transition metal complexes with Group-13 diyl ligands ER (E=BTl); (c) transition metal complexes with phosphane ligands (CO)5TMPX3 (TM=Cr, Mo, W; X=H, Me, F, Cl); (d) main group complexes with phosphane ligands X3BPY3 and X3AlPY3 (X=H, F, Cl; Y=F, Cl, Me, CN); (e) transition metal metallocene complexes Fe(η5-E5)2 and FeCp(η5-E5) (E=CH, N, P, As, Sb); (f) main group metallocenes ECp2 (E=BeBa, Zn, SiPb) and ECp (E=LiCs, BTl).

Homogeneous Gold Catalysis: Mechanism and Relativistic Effects of the Addition of Water to Propyne

Homogeneous catalysis employing gold compounds is a rapidly developing field. Au(III) catalysts in particular are interesting, since they exhibit catalytic properties unseen in other metals. In this study we report for the first time the complete mechanism of the nucleophilic addition of water to triple bonds that have not specifically been activated. The effect that the coordination of solvent molecules has on the course of the catalytic cycle is demonstrated, and the importance of hydrogen bonds to guide the substrate through the mechanism is highlighted. The influence of relativistic effects, which are particularly important for very heavy metals such as gold, is investigated, and it is concluded that the catalytic activity of gold could be seen as a relativistic effect.

Iron Bispentazole Fe(η5-N5)2, a Theoretically Predicted High-Energy Compound: Structure, Bonding Analysis, Metal–Ligand Bond Strength and a Comparison with the Isoelectronic Ferrocene

Quantum-chemical calculations with gradient-corrected (B3LYP) density functional theory have been carried out for iron bispentazole and ferrocene. The calculations predict that Fe(eta5-N5)2 is a strongly bonded complex which has D5d symmetry. The theoretically predicted total bond energy that yields Fe in the 5D ground state and two pentazole ligands is Do = 109.0 kcal mol(-1), which is only 29 kcal mol(-1) less than the calculated bond energy of ferrocene (Do = 138.0 kcal mol(-1); experimental: 158 +/- 2 kcal mol(-1)). The compound Fe(eta5-N5)2 is 260.5 kcal mol(-1) higher in energy than the experimentally known isomer Fe(N2)5, but the bond energy of the latter (Do = 33.7 kcal mol(-1)) is much less. The energy decomposition analyses of Fe(eta5-N5)2 and ferrocene show that the two compounds have similar bonding situations. The metal-ligand bonds are roughly half ionic and half covalent. The covalent bonding comes mainly from (e1g) eta5-N5- --> Fe2+ pi-donation. The previously suggested MO correlation diagram for ferrocene is nicely recovered by the Kohn-Sham orbitals. The calculated vibrational frequencies and IR intensities are reported.

Characterization of agostic interactions in theory and computation

Abstract Agostic interactions are covalent intramolecular interactions between an electron deficient metal and a σ -bond in close geometrical proximity to the metal atom. While the classic cases involve CH σ -bonds close to early transition metals like titanium, many more agostic systems have been proposed which contain CH, SiH, BH, CC and SiC σ -bonds coordinated to a wide range of metal atoms. Recent computational studies of a multitude of agostic interactions are reviewed in this contribution. It is highlighted how several difficulties with the theoretical description of the phenomenon arise because of the relative weakness of this interaction. The methodology used to compute and interpret agostic interactions is presented and different approaches such as atoms in molecules (AIMs), natural bonding orbitals (NBOs) or the electron localization function (ELF) are compared and put into context. A brief overview of the history and terminology of agostic interactions is given in the introduction and fundamental differences between α , β and other agostic interactions are explained.

The nature of the chemical bond in the light of an energy decomposition analysis

Publisher Summary This chapter summarizes the results of quantum chemical calculations where it has investigated the nature of the chemical bond in main-group and transition metal compounds with an energy decomposition analysis (EDA). The EDA decomposes the instantaneous interaction energy A –B between two fragments A and B into three terms that can be interpreted in a chemically meaningful way. The three terms are the quasi-classical electrostatic interaction between the frozen charges of the fragments ΔE elstat , the exchange (Pauli) repulsion between electrons possessing the same spin ΔE Pauli, , and the orbital interaction term ΔE orb . The latter term can be divided into contributions of orbitals having different symmetry, which allows an estimate of the strength of s, p, and d bonding. The results show that the quasi-classical electrostatic interaction significantly contributes to the bonding interactions in all molecules. The trend of the bond strength is in most cases correctly predicted by the orbital term ΔE orb but there are cases where the electrostatic attraction or the Pauli repulsion is more important for an understanding of the bonding interactions. The EDA is an unambiguously defined partitioning scheme that considers all terms yielding a chemical bond. The EDA can be considered as a bridge between the classical heuristic bonding models of chemistry and the physical mechanism of chemical bond formation.

Isolation and Characterization of a Bismuth(II) Radical

More than 80 years after Paneths report of dimethyl bismuth, the first monomeric Bi(II) radical that is stable in the solid state has been isolated and characterized. Reduction of the diamidobismuth(III) chloride Bi(NON(Ar))Cl (NON(Ar)=[O(SiMe2NAr)2](2-); Ar=2,6-iPr2C6H3) with magnesium affords the Bi(II) radical ˙Bi(NON(Ar)). X-ray crystallographic measurements are consistent with a two-coordinate bismuth in the +2 oxidation state with no short intermolecular contacts, and solid-state SQUID magnetic measurements indicate a paramagnetic compound with a single unpaired electron. EPR and density functional calculations show a metal-centered radical with >90% spin density in a p-type orbital on bismuth.

The adsorption of CO on charged and neutral Au and Au2: A comparison between wave-function based and density functional theory

Quantum theoretical calculations are presented for CO attached to charged and neutral Au and Au(2) with the aim to test the performance of currently applied density functional theory (DFT) by comparison with accurate wave-function based results. For this, we developed a compact sized correlation-consistent valence basis set which accompanies a small-core energy-consistent scalar relativistic pseudopotential for gold. The properties analyzed are geometries, dissociation energies, vibrational frequencies, ionization potentials, and electron affinities. The important role of the basis-set superposition error is addressed which can be substantial for the negatively charged systems. The dissociation energies decrease along the series Au(+)-CO, Au-CO, and Au(-)-CO and as well as along the series Au(2)(+)-CO, Au(2)-CO, and Au(2)(-)-CO. As one expects, a negative charge on gold weakens the carbon oxygen bond considerably, with a consequent redshift in the CO stretching frequency when moving from the positively charged to the neutral and the negatively charged gold atom or dimer. We find that the different density functional approximations applied are not able to correctly describe the rather weak interaction between CO and gold, thus questioning the application of DFT to CO adsorption on larger gold clusters or surfaces.

Kinetic and Thermodynamic Stability of the Group 13 Trihydrides

The kinetic and thermodynamic stabilities of the group 13 hydrides EH(3) (E = B, Al, Ga, In, Tl, E113) are investigated by relativistic density functional and wave function based theories. The unimolecular decomposition of EH(3) --> EH + H(2) becomes energetically more favorable going down the Group 13 elements, with the H(2)-abstraction of InH(3), TlH(3), and (E113)H(3) (E113: element with nuclear charge 113) being exothermic. In accordance with the Hammond-Leffler postulate, the activation barrier for the dissociation process decreases accordingly going down the group 13 elements in the periodic table shifting to an early transition state, with activation energies ranging from 88.4 kcal/mol for BH(3) to 41.3 kcal/mol for TlH(3) and only 21.6 kcal/mol for (E113)H(3) at the scalar relativistic coupled cluster level of theory. For both TlH(3) and (E113)H(3) we investigated spin-orbit effects using Dirac-Hartree-Fock and second-order Møller-Plesset theory to account for electron correlation. For (E113)H, spin-orbit coupling results in a chemically inert closed 7p(1/2)-shell, thus reducing the stability of the higher oxidation state even further. We also investigated the known organothallium compound Tl(CH(3))(3), which is thermodynamically unstable similar to TlH(3), but kinetically very stable with an activation barrier of 57.1 kcal/mol.

Accurate density functional theory description of binding constants and NMR chemical shifts of weakly interacting complexes of C60 with corannulene-based molecular bowls

Density functional calculations on “catch and release” complexes of C60 with corannulene derived molecular bowls show that computationally obtained 1H nuclear magnetic resonance (NMR) chemical shifts can be used as a reliable predictor of binding constants. A wide range of functionals was benchmarked against accurate ab initio calculations to ensure a credible representation of the weak forces that dominate the interactions in these systems. The most reliable density functional theory (DFT) results were then calibrated using experimentally observed NMR data. Careful analysis and comparison of a wide range of commonly used density functionals shows that the explicit inclusion of dispersion corrections is currently the only reliable way to accurately describe the systems investigated in our study. Moreover, we are able to show that the B97‐D and ωB97X‐D functionals are not only able to reproduce ab initio benchmark calculations, but they do so accurately with a moderately sized basis sets and without the problems of numerical integration we encountered with other functionals in this study.

Chemical bonding in octahedral XeF6 and SF6

Quantum chemical density functional theory calculations have been carried out for octahedral XeF6 and SF6 at the BP86/TZ2P level with relativistic effects included by the ZORA approximation. The energy decomposition analysis of XeF6 and SF6 using neutral and charged fragments EF5 + F and EF5+ + F− as well as E + F6 and E6+ + F66− indicates that the dominant E–F orbital interactions take place between σ-orbitals which have t1u symmetry in the octahedral point group. The contribution of the a1g orbitals is negligible in the 16 valence electron compound XeF6. The a1g contribution becomes larger in the 14 valence electron species SF6 but it is less important than the t1u term. The bonding between the neutral species comes mainly from covalent (orbital) interactions but the quasiclassical electrostatic attraction significantly contributes to the attractive interactions. The bonding which comes from the ΔEorb term is compensated by the Pauli repulsion ΔEPauli. The sum of ΔEorb and ΔEPauli is repulsive for XeF6 and SF6, which would not be stable molecules without quasiclassical electrostatic attraction.