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The Electronic Factors Governing the Relative Stabilities of Geometric Isomers of Octahedral Complexes with π-acceptor and π-donor Ligands
c29
Journal of Organometallic Chemistry, 179 (1979) C29-C33 0 Ekevier Sequoia S-A., Lausanne --Printed in The Netherlands
Preliminary
communication --_-____
The Electronic
..__ _______._.-
Factors Governing
of Octahedral
Complexes
By D. Michael lnorgonic (Great
Laboratory,
Chemistry
the Relative
-~_---
Stabilities
with i;-acceptor
-_-
of Geometric
and<-donor
_.-_ Isomers
Ligands
P. Mingos University of Oxford,
South Parks Road,
Oxford
OXi
Britain)
(Received
August 9th,
1979)
Summary
Extended Hz&e1 molecular orbitol calculations have shown that the relative stabilities
of geometric
isomers in 16, I7 and I8 electron octahedral
complexes containing strong K-acid or rr-donor ligands depend on the number of valence
g-donors
electrons
fashion on whether the ligonds ore
or 6occeptors.
Chemical
Colton
and in a complementary
(I),
and electrochemical
studies,
particularly
have given rise to the interesting observation
i8 electron complexes M(C0)2(diphos)2
by Bond and
that although
in the
(M = Cr, Mo or W) and Mn(CO)3(diphos)X(y)
the more stable isomer is that which has mutually cis- carbonyl ligands, in the related
I7 electron
dynamically
complexes
preferred.
it is the trans- and mer-
For octahedral dioxo-complexes
isomers which are thermo-
it has been noted that
it is trans- isomer which is more stable when the electron count is 18, e.g.
3OR
c30
z t
E
vv
1
c31
complexes
isomer in I6 electron
Extended completed cationic
Hbdeel
molecular
on Mo(CO)2(PH3)4,
species
indicated
calculations (5) which hove been
orbital
Mo(CO)3(PH3)3,
Mo02(PH3)4
not only have shown that this phenomenon
in terms of the occupancies
of the highest occupied
that the complementary
ligonds such OS CO preferred stability
and <-donor order
ligands such as 0
for 16 and 18 complexes
Sum of occupied
---
I electron
energy
2-
leads to a reversal of the
of these ligonds.
!
levels
for some carbonyl
Energy (eV)
-950.852
E-M~(CO)~(PH~)~
18
-950.475
-cis-Mo(CO),(PH,)4+
17
-957.664
trans-Mo(C0)2(PH3)4+
17
-957 -798
-cis-Mo02(PH3)4
18
-815.932
F-Mo02(
18
-816.492
16
-795.365
16
-794.892
PH3J
ci&Ao02(PH3)42+ 2+
O)3(PH3)3
fac-Mo(CO)3(PH3)3
fac-Mo(CO)3(PH3)3
and oxo-complexes
+ +
Stable
Isomer
Energy Diff.
Electrons
18
mer-Mo(C0)3(PH3)3
but also
observed for-r
cis-Mo(C0)2(PH3)4
mer-Mo(C
orbitals,
--No. of Valence
Compound
E-Mo02(PH3)4
and the related
has a simple explanation
molecular
nature of the effects
Table
such as
18
-1054.265
18
-1054.977
17
-1054.442
li
-1054.297
-cis-
0.377
trans-
6.134
trans-
0.560
cis-
0.473
fac-
0.712
mer-
0.145
(eV)
C32
Table
1 summarises the results of the molecular
and demonstrates stability
that this type of calculation
order for I8 and 17 electron
complexes,
molecular
metal and are derived and their energies combinations
from the t
29
and ~(0)
fashion
ce=
carbonyl
isomers are equally
more detailed stable
complex,
calculations
for both
the ligond-ligand a significant
stabilization
ligands
Figure 2 illustrates the relative detailed molecular electron
stabilities
calculations
I which
on the
of the leads to
from the angular
These suggest that in the
with
model,
the cis- and the results
the cis-
arises because
stabilities
of the
isomer to be more
of the neglect
of
which can result in
order is predicted
is accounted
) 34
in the Figure for each
dioxo-complex
predicts
(PH
Furthermore,
ligands with short metal-ligand
bond lengths. by both models,
of the isomeric possibilities
for.
that similar arguments may be used to account
have also demonstmted
isomers of Mo(C0)3(PH tint the barrier
of the isomeric possibilities
complexes
orbitols.
is at variance
of the mer- and fat-
interconversion carbonyl
which
nature of the relative
and Gdonor
29
interaction
configurations.
added the correct stability
and the complementary
2
Mo(PH3)6,
component
approximately
in the angular overlap for _cis-
Mo(C0)
occupations
between the linear
and is given
This difference
overlaps
With this co rrection
forx-acceptor
in Toble
complexes.
of the t
ond I6 electron
stable,
Sir,
the <*(CO)
p&5
isomer and for 16, 17 and I8 electron I8 electron
for -cis- and -trans-
can be estimated
expression (6),
oxo-
and orbItal
set of the parent octahedron,
a destabilization
the extent of this intemction model
the energies
and the appropriate
In the conventional
a stabilization
the observed
reflects
and I8 and 16 electron
the extent of overlap,
reflect
calculations
orbitols are localised predominantly
of the ligand Sorbitals
metal t set. 29
trans-
carbonyl
orbitals
The molecular
and Mo02(PH3)4-
overlap
accurately
Figure I represents schematically
of the highest occupied
orbital
3 )3_
for More
for the intra-
is reduced
in the I7
compared with those for the I8 electron
complexes,
Figure
2
thereby
Stabilisation
accounting
and Il’-acceptor
isomer
prediction
the complexes
is preferred
and acceptor In addition
and fat-
isomerisation
ligands
complexes
Mo(CO)3(PH3)3.
of complexes
for both
ligands
ligond
such as
within
the t
complexes
and this leads to the interesting
exceptional
+
, having
both
stabilities.
that in the case of the neutral
set of orbitals
29
, to be detected
complexes
types,
suggest
preferences
for 17 electron
AAzx(CO)~O~L~
, may have
the calculations
the geometric
are in conflict,
of the type e-
the splittings
in the irons-
facile
for 16 and 18 electron
the trans-
complexes
in mer-
and Mn(C0)3(diphos)X.f
Although
-K -donor
set
2g
for the observed
cis-Cr(C0)2(diphosji
for the e-donor
of the t
by W
is sufficiently
photoelectron
large,
spectral
carbonyl especially
studies_
Acknowledgement -_The S.R.C.
is thanked
for financial
support_
References l_
A.M.
Bond,
R_Colton
2.
A.M.
Bond,
R. Colton
3.
D. Grandjean
4.
V.W.
5.
R. Hoffmann,
6.
J.
D ay,
K. Burdett,
and J.J.
Jackowski,
Inorg.
and
J.M.
McCormick,
and R. Weiss,
Bull.
Sot.
and J.L.
Hoard,
J’. Chem. Inorg.
Phys.,
Chem.,
J_ Amer. g, 14,
Chim. Chem.
1397,1963. 375,
1975.
Chem.,
Inorg.
2
Chem.,
, 274, 2,
1975. 155,
(France),
3049,
1967.
Sot.
3374,
1968.
,s,
1977.
Journal of Organometallic Chemistry, 179 (1979) C29-C33 0 Ekevier Sequoia S-A., Lausanne --Printed in The Netherlands
Preliminary
communication --_-____
The Electronic
..__ _______._.-
Factors Governing
of Octahedral
Complexes
By D. Michael lnorgonic (Great
Laboratory,
Chemistry
the Relative
-~_---
Stabilities
with i;-acceptor
-_-
of Geometric
and<-donor
_.-_ Isomers
Ligands
P. Mingos University of Oxford,
South Parks Road,
Oxford
OXi
Britain)
(Received
August 9th,
1979)
Summary
Extended Hz&e1 molecular orbitol calculations have shown that the relative stabilities
of geometric
isomers in 16, I7 and I8 electron octahedral
complexes containing strong K-acid or rr-donor ligands depend on the number of valence
g-donors
electrons
fashion on whether the ligonds ore
or 6occeptors.
Chemical
Colton
and in a complementary
(I),
and electrochemical
studies,
particularly
have given rise to the interesting observation
i8 electron complexes M(C0)2(diphos)2
by Bond and
that although
in the
(M = Cr, Mo or W) and Mn(CO)3(diphos)X(y)
the more stable isomer is that which has mutually cis- carbonyl ligands, in the related
I7 electron
dynamically
complexes
preferred.
it is the trans- and mer-
For octahedral dioxo-complexes
isomers which are thermo-
it has been noted that
it is trans- isomer which is more stable when the electron count is 18, e.g.
3OR
c30
z t
E
vv
1
c31
complexes
isomer in I6 electron
Extended completed cationic
Hbdeel
molecular
on Mo(CO)2(PH3)4,
species
indicated
calculations (5) which hove been
orbital
Mo(CO)3(PH3)3,
Mo02(PH3)4
not only have shown that this phenomenon
in terms of the occupancies
of the highest occupied
that the complementary
ligonds such OS CO preferred stability
and <-donor order
ligands such as 0
for 16 and 18 complexes
Sum of occupied
---
I electron
energy
2-
leads to a reversal of the
of these ligonds.
!
levels
for some carbonyl
Energy (eV)
-950.852
E-M~(CO)~(PH~)~
18
-950.475
-cis-Mo(CO),(PH,)4+
17
-957.664
trans-Mo(C0)2(PH3)4+
17
-957 -798
-cis-Mo02(PH3)4
18
-815.932
F-Mo02(
18
-816.492
16
-795.365
16
-794.892
PH3J
ci&Ao02(PH3)42+ 2+
O)3(PH3)3
fac-Mo(CO)3(PH3)3
fac-Mo(CO)3(PH3)3
and oxo-complexes
+ +
Stable
Isomer
Energy Diff.
Electrons
18
mer-Mo(C0)3(PH3)3
but also
observed for-r
cis-Mo(C0)2(PH3)4
mer-Mo(C
orbitals,
--No. of Valence
Compound
E-Mo02(PH3)4
and the related
has a simple explanation
molecular
nature of the effects
Table
such as
18
-1054.265
18
-1054.977
17
-1054.442
li
-1054.297
-cis-
0.377
trans-
6.134
trans-
0.560
cis-
0.473
fac-
0.712
mer-
0.145
(eV)
C32
Table
1 summarises the results of the molecular
and demonstrates stability
that this type of calculation
order for I8 and 17 electron
complexes,
molecular
metal and are derived and their energies combinations
from the t
29
and ~(0)
fashion
ce=
carbonyl
isomers are equally
more detailed stable
complex,
calculations
for both
the ligond-ligand a significant
stabilization
ligands
Figure 2 illustrates the relative detailed molecular electron
stabilities
calculations
I which
on the
of the leads to
from the angular
These suggest that in the
with
model,
the cis- and the results
the cis-
arises because
stabilities
of the
isomer to be more
of the neglect
of
which can result in
order is predicted
is accounted
) 34
in the Figure for each
dioxo-complex
predicts
(PH
Furthermore,
ligands with short metal-ligand
bond lengths. by both models,
of the isomeric possibilities
for.
that similar arguments may be used to account
have also demonstmted
isomers of Mo(C0)3(PH tint the barrier
of the isomeric possibilities
complexes
orbitols.
is at variance
of the mer- and fat-
interconversion carbonyl
which
nature of the relative
and Gdonor
29
interaction
configurations.
added the correct stability
and the complementary
2
Mo(PH3)6,
component
approximately
in the angular overlap for _cis-
Mo(C0)
occupations
between the linear
and is given
This difference
overlaps
With this co rrection
forx-acceptor
in Toble
complexes.
of the t
ond I6 electron
stable,
Sir,
the <*(CO)
p&5
isomer and for 16, 17 and I8 electron I8 electron
for -cis- and -trans-
can be estimated
expression (6),
oxo-
and orbItal
set of the parent octahedron,
a destabilization
the extent of this intemction model
the energies
and the appropriate
In the conventional
a stabilization
the observed
reflects
and I8 and 16 electron
the extent of overlap,
reflect
calculations
orbitols are localised predominantly
of the ligand Sorbitals
metal t set. 29
trans-
carbonyl
orbitals
The molecular
and Mo02(PH3)4-
overlap
accurately
Figure I represents schematically
of the highest occupied
orbital
3 )3_
for More
for the intra-
is reduced
in the I7
compared with those for the I8 electron
complexes,
Figure
2
thereby
Stabilisation
accounting
and Il’-acceptor
isomer
prediction
the complexes
is preferred
and acceptor In addition
and fat-
isomerisation
ligands
complexes
Mo(CO)3(PH3)3.
of complexes
for both
ligands
ligond
such as
within
the t
complexes
and this leads to the interesting
exceptional
+
, having
both
stabilities.
that in the case of the neutral
set of orbitals
29
, to be detected
complexes
types,
suggest
preferences
for 17 electron
AAzx(CO)~O~L~
, may have
the calculations
the geometric
are in conflict,
of the type e-
the splittings
in the irons-
facile
for 16 and 18 electron
the trans-
complexes
in mer-
and Mn(C0)3(diphos)X.f
Although
-K -donor
set
2g
for the observed
cis-Cr(C0)2(diphosji
for the e-donor
of the t
by W
is sufficiently
photoelectron
large,
spectral
carbonyl especially
studies_
Acknowledgement -_The S.R.C.
is thanked
for financial
support_
References l_
A.M.
Bond,
R_Colton
2.
A.M.
Bond,
R. Colton
3.
D. Grandjean
4.
V.W.
5.
R. Hoffmann,
6.
J.
D ay,
K. Burdett,
and J.J.
Jackowski,
Inorg.
and
J.M.
McCormick,
and R. Weiss,
Bull.
Sot.
and J.L.
Hoard,
J’. Chem. Inorg.
Phys.,
Chem.,
J_ Amer. g, 14,
Chim. Chem.
1397,1963. 375,
1975.
Chem.,
Inorg.
2
Chem.,
, 274, 2,
1975. 155,
(France),
3049,
1967.
Sot.
3374,
1968.
,s,
1977.