Chemistry Reference
In-Depth Information
orbitals of their silane and metal fragment moieties led Mc Grady et al. [
84
]to
formulate a new and more sophisticated Mn (
2
-SiH) bonding model, improving
and replacing the previously established
*(Si-
X
) mechanism to
explain the electronic influence of the auxiliary
X
ligand
trans
to the activated
Si-H bond. It was found that the Mn (
s
(Mn-H)
! s
2
-SiH) bonding in all of these systems occurs
through an asymmetric oxidative addition reaction coordinate in which the Mn-H
bond is formed at an early stage, whereas Mn-Si bonding is being controlled and
enforced by the extent of Mn
-back donation. This electron flow
from a filled metal orbital of proper symmetry into a three-center ligand orbital
displaying both Si-
X
and Si-H antibonding character results in a simultaneous
activation of both the
! s
*(
X
-Si-H)
p
2
-coordinating Si-H bond and the Si-
X
bond in
trans
position. As a consequence, the control role exerted by the
trans-
oriented
X
ligand
on the geometry of the Mn(
2
-SiH) moiety becomes manifest: the higher the
electron-withdrawing character of
X
, the greater the Si-
X
and Si-H bond activation,
owing to the increased Mn
-back donation. This MO model interpreta-
tion (clearly available only at a theoretical level) was, however, unequivocally
corroborated by an SF analysis on both the experimental and theoretical charge
densities of the three investigated complexes. Figure
13d-f
displays S% contribu-
tions from atomic basins to the electron density at different bcps of the Mn(
!
ligand
p
2
-
HSiFPh
2
) moiety. When the rp is at the Mn-H bcp (Fig.
13d
), the dominant
contributions come from the Mn atom (27%;
29
%, experimental values in
italic
)
and the H atom (39%;
39
%), whereas the contribution of Si is indeed small (3%;
3
%) as expected for a strongly localized Mn-H bond. Note also that almost
unchanged percentage contributions are found for the other two studied complexes.
This result confirms that the oxidative addition of the silane ligand to Mn is an
asymmetric process in which the Mn-H is formed at an early stage and that such
bond, as expected, cannot be significantly affected by the extent of the Mn
! s
*
(
X
-Si-H)
-back donation. The situation appears quite different when the rp is
placed at the Mn-Si bcp (Fig.
13e
). Here Mn, Si, and H each contribute to a very
similar extent to the bcp density (13%, 19%, 18%;
16
%,
21
%,
15
%, for Mn, Si, and
H, respectively, and with experimental values in
italic
). Such delocalized sources
imply a strongly delocalized interaction, with formation of the Mn-Si bond affect-
ing directly the Si-H bond (hence the H atom percentage contribution) because of
the
p
*(
X
-Si-H) orbital.
Analogously, when the rp is placed at the Si-H bcp (Fig.
13f
), very delocalized
sources are again observed, with a nonnegligible contribution from the Mn atom.
According to the Mn
p
-back donation from the metal into the antibonding
s
!s
*(
X
-Si-H) model, the S%(Mn) was found to theoretically
increase from
X
Cl (9%), confirming that the
back-donation from Mn to Si increases as the Si center becomes more positive.
The SF analysis “translates” to a set of percentage atomic contributions the
information in Fig.
13c
, where the Laplacian distribution of the density in the
Mn-H-Si-F plane, overlaid with the bond paths within the Mn(
¼
H (5%), to
X
¼
F (6%) and to
X
¼
2
-HSiF) moiety,
is displayed. It is gratifying to see that the SF provides not only a precious chemical
insight into an intriguing bonding scenario, but also a physical validation, both at a
theoretical and at an experimental level, of the MO model interpretation of bonding
