title: Nature of the bonding in metal-silane sigma-complexes.
abstract: 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)(eta(2)-HSiHPh(2))] 1 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) A, and H(1)-Si-H(2) 148.5(8) degrees. This complex is similar to [Cp'Mn(CO)(2)(eta(2)-HSiFPh(2))] 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 T(1) relaxation measurements, giving a value of 1.56(3) A 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 [Cp'Mn(CO)(2)(eta(2)-HSiCl(3))] 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 eta(2)-coordinated Si-H moiety are controlled by the extent of Mn --> 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(eta(2)-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.
authors.affiliation: Department of Chemistry, University of New Brunswick, P.O. Box 4400, Fredericton, N.B. E3B 5A3, Canada. smcgrady@unb.ca
authors.forename: G Sean
authors.initials: GS
authors.lastname: McGrady
authors.forename: Peter
authors.initials: P
authors.lastname: Sirsch
authors.forename: Nicholas P
authors.initials: NP
authors.lastname: Chatterton
authors.forename: Andreas
authors.initials: A
authors.lastname: Ostermann
authors.forename: Carlo
authors.initials: C
authors.lastname: Gatti
authors.forename: Sandra
authors.initials: S
authors.lastname: Altmannshofer
authors.forename: Verena
authors.initials: V
authors.lastname: Herz
authors.forename: Georg
authors.initials: G
authors.lastname: Eickerling
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Scherer
mesh_terms: D056831:Coordination Complexes / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D033363:Neutron Diffraction; D012821:Silanes / Q000737:chemistry*
chemical_list: D056831:Coordination Complexes; D012821:Silanes