On the Nature of the Bonding in Metal-Silane sigma-Complexes

TL;DR

This study investigates the bonding in metal-silane sigma-complexes using experimental structures, NMR measurements, and DFT calculations, revealing insights into the bonding mechanism and geometric features of these complexes.

Contribution

It provides detailed structural, spectroscopic, and computational analysis of metal-silane sigma-complexes, highlighting the asymmetric oxidative addition process and the role of back-donation.

Findings

The Si atom in complexes has a trigonal bipyramidal geometry.

Mn-H distances in solution match neutron diffraction data.

Back-donation influences the activation of Si-H bonds.

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(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).