  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).
