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The nature of metal silane sigma-bond interaction has been investigated in several key systems by a range of experimental and computational techniques. The structure of [Cp'Mn(CO)2(eta2-HSiHPh2)] 1 has been determined by single crystal neutron diffraction, and the geometry at the Si atom is shown to approximate to a trigonal bipyramid. This complex is similar to [Cp'Mn(CO)2(eta2-HSiFPh2)] 2, whose structure and bonding characteristics have recently been determined by charge density studies based on high-resolution X-ray and neutron diffraction data. The geometry at the Si atom in these sigma-bond complexes is compared with that in other systems containing hypercoordinate silicon. The Mn-H distances for 1 and 2 in solution have been estimated using NMR T1 relaxation measurements, giving a value of 1.56(3) AA in each case, in excellent agreement with the distances deduced from neutron diffraction. DFT calculations have been employed to explore the bonding in the Mn-H-Si unit in 1 and 2 and in the related system [Cp'Mn(CO)2(eta2-HSiCl3)] 3. These studies support the idea that the oxidative addition of a silane ligand to a transition metal center may be described as an asymmetric process in which the Mn-H bond is formed at an early stage, while both the establishment of the Mn-Si bond and also the activation of the eta2-coordinated Si-H moiety are controlled by the extent of Mn-> sigma*(X-Si-H) back-donation, which increases with increasing electronwithdrawing character of the X substituent trans to the metal-coordinated Si-H bond. This delocalized molecular orbital (MO) approach is complemented and supported by combined experimental and theoretical charge density studies including the source function S(r,Omega)., Comment: 39 pages, 7 figures, 4 tables

The molecular structures of the monomeric, pentacoordinated ethylchloroniobium(V) compounds Me3NbCl2 and Me2NbCl3 have been determined by gas electron diffraction (GED)and Density Functional Theory (DFT) calculations, and, for Me3NbCl2, by single crystal X-ray diffraction. Each of the molecules is found to have a heavy-atom skeleton in the form of a trigonal bipyramid (TBP) with Cl atoms in the axial positions, in accord with their vibrational spectra. The TBP is somewhat distorted in the case of Me2NbCl3 with the two axial Nb-Cl bonds bent away from the equatorial, slightly shorter Nb-Cl bond. In the case of Me3NbCl2, moreover, the X-ray model suggests structural distortions away from the idealized C3h geometry, in line with the results of quantum chemical calculations. The electronic structures of Me2NbCl3 and Me3NbCl2 have been explored by rigorous analysis of both the wavefunction and the topology of the electron density, employing DFT calculations. Hence the structures of these compounds are shown to reflect repulsion between the Nb-C and Nb-Cl bonding electron density and charge concentrations induced by the methyl ligands in the valence shell of the Nb atom and arising mainly from use of Nb(4d) functions in the Nb-C bonds., Comment: 32 pages, 8 figures, submitted to Chem. Eur. J