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The ²⁹Si chemical shifts in substituted disilynes have been analyzed by quantum-chemical studies using a detailed breakdown of paramagnetic contributions into couplings of occupied and virtual canonical molecular orbitals. The results give indications of substituent effects on shielding and confirm the importance of energy denominators in the equation for σ^p in symmetrical substituted disilynes (H3SiSiSiSiH3 and H3CSiSiCH3) on their chemical shifts. In the unsymmetrical substituted disilyne H3CSiSiSiH3 the energy denominators are identical for both sites and thus cannot explain the widely different chemical shifts. It turns out that the asymmetric charge distribution (reflected in both occupied and virtual MOs and sampled by the PSO matrix elements) allows more efficient couplings at the methyl-substituted silicon side. Furthermore, shielding values in disilynes depend strongly on the size of the trans-bent angle ϕ, as indicated clearly by analyses of the ²⁹Si chemical shifts in the model H3SiSiSiSiH3 at selected values of ϕ. Small changes in ϕ influence the ²⁹Si shielding values appreciably. At lower values of ϕ, couplings between bonding and virtual MOs exhibit stronger deshielding caused by significantly lower energy denominators.

Previous ²⁹Si NMR spectroscopic investigations of various heteroatom-substituted silyllithium compounds revealed “unexpectedly” high silicon chemical shifts. To find explanations for these observations, the ²⁹Si chemical shift tensors of various methoxy-, dimethylamino-, and methylthio-substituted chloro and lithiosilanes have been evaluated by quantum-chemical calculations. Substituent effects on shielding have been analyzed by IGLO-DFT calculations. Popular notions on the influence of atomic charge and substituent electronegativity on the central-atom shifts are not applicable. The combination of very electronegative and electropositive substituents leads to large deshielding contributions in heteroatom-substituted silyllithium compounds. As shown by analyses of localized molecular orbital (LMO) contributions to shielding, this is due to a coupling of bonding and antibonding orbitals with large silicon character and small energy differences. After a comparison of IGLO-LMO results and GIAO-based natural chemical shielding analyses, the LMO interpretations have been augmented further by an inspection of natural bond orbitals, to provide a consistent interpretation of the substituent effects on chemical shifts. The presented results should be transferable also to other heteroatom-substituted p-block main-group compounds.

Reaction of trimethoxy(morpholinomethyl)silane (7) with lithium aluminum hydride yielded (morpholinomethyl)silane (8). Treatment of 8 with ethane-1,2-dithiol (molar ratio 1:2) in MeCN afforded bis[ethane-1,2-dithiolato(2−)](morpholiniomethyl)silicate (5; isolated as 5·MeCN). Reaction of 7 with ethane-1,2-diol (molar ratio 1:2) in MeCN yielded the oxygen analogue bis[ethane-1,2-diolato(2−)](morpholiniomethyl)silicate (6). Single-crystal X-ray diffraction studies showed that the Si-coordination polyhedra of 5·MeCN and 6 are distorted trigonal bipyramids. Compounds 5·MeCN and 6 were additionally characterized by solid-state VACP/MAS NMR experiments (¹³C, ²⁹Si). The structural investigations were complemented by quantum-chemical studies of the structure, bonding, and ²⁹Si NMR chemical shift tensors of 5 (SiS4C skeleton) and 6 (SiO4C skeleton).

The ²⁹Si chemical shift tensors in both symmetrically and unsymmetrically substituted disilenes have been analyzed by quantum chemical calculations. A detailed breakdown of the paramagnetic contributions to nuclear shielding into individual sum-over-states terms allows new insights into the interrelations between bonding and chemical shifts in these unsaturated compounds. While the substituent effects in symmetrically substituted systems may be understood to a large extent from the energy denominators of the perturbation expression, the very different shifts for the two silicon nuclei in unsymmetrically substituted systems are mainly due to the spatial extent and orientation of the occupied and unoccupied molecular orbitals, as sampled by the matrix elements of nuclear magnetic moments at both silicon sites. The unequal shifts may thus be understood and could serve as a diagnostic tool to distinguish between symmetrically and unsymmetrically substituted disilenes. Furthermore, unequal charge transfer and an alternating charge distribution can rationalize unexpectedly larger pyramidalization of the silyl-substituted silicon center in unsymmetrically substituted systems.

The reaction of open and half-open sandwich complexes of iron and ruthenium with metal carbonyls was studied. Three types of complexes could be isolated:  Carbonyl-bridged iron−ruthenium dimers with cyclopentadienyl and pentadienyl terminal ligands, dimetallic iron carbonyl complexes with a bridging σ,π-coordinated pentadienyl ligand, and ruthenium complexes with one or two π-coordinated ruthenabenzene ligands. The latter are the first ever isolated bis(metallabenzene) π-complexes. The X-ray structures of C5Me5Ru(CO)4Fe(C7H11), (μ-C7H10)Fe2(CO)6, and [(C8H11)Ru(CO)3]2Ru are reported. Density functional calculations have been used to better understand the bonding in metallabenzene complexes. The electron localization function (ELF) clearly confirms metal−metal bonding interactions in the complexes studied. In particular, the bis(metallabenzene) complex 13 is found to exhibit a three-center bonding attractor in the ELF in its stable syn-eclipsed conformation but two two-center attractors in the ca. 30 kJ mol^-1 less stable anti-eclipsed arrangement. The frontier orbitals of metallabenzenes exhibit very large participation of the ring metal. When metallabenzenes act as ligands to other metal fragments, direct metal−metal interactions are thus expected to be a general feature of the complexes, unless they are replaced by other interactions, e.g. by protonation. Natural population analyses suggest that the σ-donor abilities of the electron-rich d⁸ metallabenzene fragments predominate over their π-acceptor character.

The influence of ortho sulfur substitution in the modified tyrosyl radical in apo-galactose oxidase on the g tensor and the spin-density distribution has been studied by calculations on various model systems using an accurate DFT approach. Computed g tensors agree well with experimental observation, and they are intermediate between the extremely large substituent effect found in a previous DFT study and the very small changes found in MCSCF calculations. The origin of the substituent effects has been studied by fragment analyses of the spin−orbit/orbital-Zeeman g-tensor contributions, and it is discussed in relation to the spin-density distributions. The influence of hydrogen bonding on g tensors is studied by appropriate model complexes with water. Further calculations on radicals including heavier chalcogen substituents XH (X = O, S, Se, Te) predict very large gx and gy components with selenium- and tellurium-substituted systems due to large, direct substituent spin−orbit contributions. In view of an unexpectedly large gauge dependence of the g-tensor orientation in several cases, a recently developed implementation of gauge-including atomic orbitals (GIAO) has been applied.

The unusual trigonal prismatic structure of tris(butadiene)molybdenum, reported in 1975 by Skell, has been revisited by extensive quantum chemical calculations and by a low-temperature single-crystal X-ray diffraction study. While a trigonal prismatic coordination arrangement is confirmed by DFT and MP2 structure optimizations, the calculations provide very different bond lengths than earlier crystallographic studies:  Due to appreciable back-bonding, the terminal Mo−C1 bonds are significantly shorter than the central Mo−C2 bonds (ca. 2.29 vs ca. 2.36 Å), and the central (C2−C2A) bonds are actually shorter than the terminal ones (ca. 1.40 vs ca. 1.44 Å), as found previously for substituted complexes. Similar structures have been computed for tris(butadiene)tungsten and for related, substituted systems. A structure redetermination of tris(butadiene)molybdenum at low temperature shows that the erroneous bond lengths obtained previously are due to the presence of a disorder resulting from the superposition of two different orientations of the three butadiene ligands with different site occupation factors. Refinement of this disorder results in a physically more plausible orientation of the anisotropic displacement parameter and gives by a factor of 10 improved estimated standard deviations for the geometrical features. A much better agreement between theory and experiment is attained. It is now obvious that resonance structures involving metallacyclopentene rings contribute significantly to bonding. This conclusion has been confirmed by natural bond orbital/natural resonance theory analyses, which indicate overall larger contributions from metallacyclopentene resonance structures than from traditional resonance structures with π-bonded diolefins. Explanations are provided for the trigonal prismatic structure preferences. MO analyses differ qualitatively and quantitatively from previous work, due to the use of refined structural parameters. Computed ligand NMR chemical shifts agree well with experimental data, provided that they are calculated at the correct structures.

Deviations of the coordination arrangement of vanadyl complexes from a regular square pyramid are thought to influence, among other things, their biological function. Such structural distortions have been found to be reflected characteristically in EPR spectra (Cornman et al. Inorg. Chem. 1997, 366, 6401). In this work, density functional calculations of electronic g tensors and metal hyperfine coupling tensors have been carried out for a series of four of these vanadyl complexes with structures ranging from nearly trigonal bipyramidal (TBP-5) to nearly square pyramidal (SQP-5). The EPR spectroscopic parameters have been rationalized in terms of electronic and geometrical structures. Using all relevant perturbation operators together with local or gradient-corrected density functionals, Δg-tensor components are underestimated systematically by ca. 40%. Good agreement with experiment is obtained for hyperfine tensor components calculated with hybrid functionals (B3PW91 and BHPW91), which account better for the spin polarization of the core orbitals than GGA functionals such as BP86. The rhombicity of the hyperfine tensor is reproduced well at all levels of theory applied. It is mainly determined by the SOMO composition. The latter explains the increasing rhombicity of the A tensor with increasing distortion of the SQP-5 structures along the series of complexes studied. The orientational dependence of the principal tensor components on the local vanadium coordination is much more pronounced for the g tensor than for the A tensor. The principal axes of the g and A tensors are found to be rotated with respect to each other by as much as 41°.

The ¹⁷O NMR chemical shift tensors for a dioxygen ligand bound to iron porphyrin model complexes, with or without inclusion of picket-fence-type substituents, have been calculated by density functional IGLO approaches, based on structures optimized in Car−Parrinello molecular dynamics simulations. The calculations confirm the experimentally found, extremely large ¹⁷O shifts, and the assignment of the higher-frequency signal to the terminal oxygen position. Metal−ligand π-back-bonding and the presence of low-lying excited states influence the chemical shift tensors characteristically. The magnitude and orientation of the shift tensors also reflect the close analogy to the bonding in the ozone molecule. Possible explanations for the temperature dependence of the solid-state NMR spectra are discussed. The computed ¹⁷O nuclear quadrupole coupling constants of ca. 11 MHz and ca. 17 MHz for bridging and terminal oxygen positions are close to the results for ozone, where theory and experiment agree well. This contradicts earlier assumptions of very small ¹⁷O field gradients for picket-fence oxyheme models and raises some questions about the analysis of the solid-state ¹⁷O NMR spectra. Possible reasons for the temperature dependence of the line widths in the solution spectra are suggested. We also discuss the implications of the ¹⁷O NMR spectra for the nature of the ground state of oxyheme complexes.

The performance of various density functional approaches for the calculation of electron paramagnetic resonance (EPR) hyperfine coupling constants in transition metal complexes has been evaluated critically by comparison with experimental data and high-level coupled-cluster results for 21 systems, representing a large variety of different electronic situations. While both gradient-corrected and hybrid functionals allow the calculation of isotropic metal hyperfine coupling constants to within ca. 10−15% for the less critical cases (e.g., ScO, TiN, TiO, VO, MnO, MnF), none of the functionals investigated performs well for all complexes. Gradient-corrected functionals tend to underestimate the important core−shell spin polarization. While this may be improved by exact-exchange mixing in some cases, the accompanying spin contamination may even lead to a deterioration of the results for other complexes. We also identify cases, where essentially none of the functionals performs satisfactorily. In the absence of a ”universal functional”, the functionals to be applied to the calculation of hyperfine couplings in certain areas of transition metal chemistry have to be carefully selected. Desirable, improved functionals should provide sufficiently large spin polarization for core and valence shells without exaggerating it for the latter (and thus introducing spin contamination). Coupling anisotropies and coupling constants for ligand nuclei are also discussed. The computationally much more demanding coupled cluster (CCSD and CCSD(T)) methods, which have been applied to a subset of complexes, show good performance, even when a UHF reference wave function is moderately spin-contaminated.

We have investigated the effect of hydrostatic pressure on the structural properties of CsGeBr₃ by means of X-ray diffraction and Raman spectroscopy. Pressures were generated using diamond anvil cells. At 1.0(2) GPa the trigonal ambient pressure modification of CsGeBr₃ undergoes a reversible first-order phase transition to a high-pressure polymorph crystallizing in the cubic perovskite structure. The variation in Ge-Br bond length is obtained from single-crystal structure refinements of both the low- and the high-pressure modifications. The phase transformation occurs at a relative volume of V/V₀ = 0.92, and the prediction from Hartree-Fock calculations is in agreement with this experimental result. Copyright 1995, 1999 Academic Press

The reaction of indium monobromide (InBr) with LiC(SiMeRR‘)3·xTHF gives in a high yield alkylindium(I) derivatives with the indium atoms in the unusual low oxidation state of +1. The properties of these products are investigated by the systematic variation of the steric demand of the C(SiMeRR‘)3 substitutents (R = Me, Et; R‘ = Me, Et, ⁿBu, ⁱPr, Ph). Tetrahedral In4 clusters are observed in the solid state and in solution for smaller substituents such as C(SiMe3)3, C(SiMe2Et)3 (1), and C(SiMe2ⁿBu)3 (2). As shown by two crystal structure determinations, those clusters exhibit undistorted tetrahedra of four indium atoms and short In−In distances of 3.00 Å on average. In contrast, owing to the larger steric stress, complete dissociation into the monomeric formula units InR is observed in solution with the more voluminous C(SiMe2ⁱPr)3 group (3), while in the solid state once more an almost undistorted tetrahedron of the tetramer is found, showing, however, much elongated In−In distances of 3.155 Å on average. Single crystals could not be obtained for the compounds with C(SiMe2Ph)3 (4) and C(SiEt2Me)3 groups (5); 4 is monomeric in benzene, while 5 gives the formula mass of the dimer, and complete dissociation into the monomer is observed only in very dilute solutions. All compounds show unusual downfield shifts of the resonances of the carbon atoms bound to indium up to δ = 76 ppm, which are very characteristic of the alkylindium(I) compounds. Quantum chemical NMR chemical shift calculations using density functional theory indicate that the large shifts are related to the presence of low-lying magnetically allowed excited states, and they are further enhanced by unusually large spin−orbit effects. Optimized structure parameters for InCH3, In4(CH3)4, InH, and In4H4 are compared with the experimental results. Our best estimate for the tetramerization energy of InCH3 is ca. 290 kJ/mol at the MP2 level.

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