Experimental and theoretical comparison between M(cp)Cl3Ln systems of NbIV and MoIV (cp = η-C5H5)
Abstract
The controlled sodium reduction of Nb(cp)Cl
4
L
(cp = η-C
5
H
5
;
L = PMe
3
, PMe
2
Ph or
PMePh
2
) or
Nb(η-C
5
Me
5
)Cl
4
in the presence of
PMe
3
afforded the mononuclear 15-electron complexes
Nb(cp)Cl
3
L and
Nb(η-C
5
Me
5
)Cl
3
(PMe
3
),
respectively. Reduction of Nb(cp)Cl
4
in the presence of an
excess of L for PMe
2
Ph and PMePh
2
afforded solids
that contain mainly the 17-electron Nb(cp)Cl
3
L
2
species but are contaminated by the mono-L derivatives. A UV/VIS
investigation of the solution equilibrium between
Nb(cp)Cl
3
(PMe
2
Ph)
2
and
Nb(cp)Cl
3
(PMe
2
Ph) plus free PMe
2
Ph
afforded an enthalpy of 19.0 ± 1.6 kcal
mol
-1
and an entropy of 45 ± 5 cal
K
-1
mol
-1
for the ligand dissociation
process. A comparative study of the equilibrium between
Mo(cp)Cl
3
(PMe
2
Ph)
2
and
Mo(cp)Cl
3
(PMe
2
Ph) plus free PMe
2
Ph
cannot be carried out because the equilibration is too slow at room
temperature and because of thermal decomposition with ring loss at high
temperature. Theoretical calculations at the second-order
Møller-Plesset perturbation (MP2) level on the
M(cp)Cl
3
(PH
3
)
n
(M = Nb or Mo, n = 1 or 2) model
systems afforded geometries in good agreement with experimental
examples. The calculated PH
3
dissociation energy for
M = Nb of 21.3 kcal mol
-1
is in good
agreement with experiment. For M = Mo, the more saturated
complex is stabilized by 32.8 kcal mol
-1
relative to
the excited
1
A′ state and by 23.5 kcal
mol
-1
relative to the ground
3
A″
state. Therefore, the regain of pairing energy upon PH
3
dissociation from Mo(cp)Cl
3
(PH
3
)
2
provides a calculated stabilization for the 16-electron monophosphine
complex of 9.3 kcal mol
-1
. The observed variations of
bonding parameters upon metal change from Nb to Mo and a natural
population analysis suggest that the main reason for a greater
Mo–PH
3
bonding interaction is the greater extent of
both M–P σ bonding and π back bonding for the
d
2
metal relative to the d
1
metal.
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