Nature of the Bonding in Metal-Silane σ-Complexes

The nature of metal silane σ-bond interaction has been investigated in several key systems by a range of experimental and computational techniques. The structure of [Cp′Mn(CO)₂(η²-HSiHPh₂)] <b>1</b> has been determined by single crystal neutron diffraction, and the geometry at the Si atom is shown to approximate a trigonal bipyramid; salient bond distances and angles are Mn−H(1) 1.575(14), Si−H(1) 1.806(14), Si−H(2) 1.501(13) Å, and H(1)−Si−H(2) 148.5(8)°. This complex is similar to [Cp′Mn(CO)₂(η²-HSiFPh₂)] <b>2</b>, 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 σ-bond complexes is compared with that in other systems containing hypercoordinate silicon. The Mn−H distances for <b>1</b> and <b>2</b> in solution have been estimated using NMR <i>T</i>₁ relaxation measurements, giving a value of 1.56(3) Å in each case, in excellent agreement with the distances deduced from neutron diffraction. Density functional theory calculations have been employed to explore the bonding in the Mn−H−Si unit in <b>1</b> and <b>2</b> and in the related system [Cp′Mn(CO)₂(η²-HSiCl₃)] <b>3</b>. 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 η²-coordinated Si−H moiety are controlled by the extent of Mn → σ*(X−Si−H) back-donation, which increases with increasing electron-withdrawing character of the X substituent <i>trans</i> 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: the source function <i>S</i>(<b>r</b>,Ω), which provides a measure of the relative importance of each atom’s contribution to the density at a specific reference point <b>r</b>, clearly shows that all three atoms of the Mn(η²-SiH) moiety contribute to a very similar extent to the density at the Mn−Si bond critical point, in pleasing agreement with the MO model. Hence, we advance a consistent and unifying concept which accounts for the degree of Si−H activation in these silane σ-bond complexes