Nature of the Bonding in Metal-Silane sigma-Complexes

The nature of metal silane ?-bond interaction has been investigated in several key systems by a range of experimental and computational techniques. The structure of [Cp?Mn(CO)2(?2-HSiHPh2)] 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(?2-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 ?-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) 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 1 and 2 and in the related system [CpetaMn(CO)2(?2-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 f sigma*(X-Si-H) back-donation, which increases with increasing electron-withdrawing 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: the source function S(r,omega), which provides a measure of the relative importance of each atom's contribution to the density at a specific reference point r, clearly shows that all three atoms of the Mn(eta2-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 sigma-bond complexes.