1. The trans effect refers to the observation that certain ligands increase the rate of ligand substitution when positioned trans to the departing ligand.
2. This effect was first discovered in 1926 when studying platinum complexes, where it was found that ammonia preferentially substituted the chloride ligand cis rather than trans to the nitrite ligand in Pt(NO2)Cl3 complexes.
3. Two main theories have been proposed to explain the trans effect - the polarization theory involving electrostatic weakening of the trans metal-ligand bond, and the pi-bonding theory involving back-donation of electron density from the metal into the pi* orbitals of ligands like NO2 weakening the trans bond.
1. 1
WHAT IS A TRANSWHAT IS A TRANS
EFFECT?EFFECT?
2. 2
INTRODUCTION
• The presence of large deposits of platinum
ores in Russia caused an intensive study of
the co-ordination compounds early in the
development of coordination chemistry.
• As a result of these studies by the Russian
School, the first displacement reaction was
discovered.
3. 3
• For example consider the substitution reaction in Pt
(II) square-planar complex [Pt(NO2)Cl3]2-
with NH3 to
yield [Pt(NO2)Cl2(NH3)]-
.Theoretically, two reaction
products are possible. These may be formed as
follows:
4. 4
• By experiments, it is found that only the trans-
isomer is formed by the replacement of Cl-
lying trans to NO2
-
in [Pt(NO2)Cl3]2-
by NH3.
• The formation of this trans-isomer has been
explained by saying that Cl-
ion lying trans to
NO2in [Pt(NO2)Cl3]2-
is replaced more easily by
NH3than either of the two Cl-
ions which are
lying cis to NO2
-
ion.The phenomenon of such
type of replacement is known as trans effect.
5. 5
• The groups like NO2
-
which control the
entering of the ligand to occupy the position
trans to them are known as trans directing
groups.
• The discovery of the trans effect is attributed
to Chernuyaev , who recognized it and gave it
a name in 1926.
6. 6
DEFINITION
• Simply, the trans effect is
the labilization (making more reactive)
of ligands that are trans to certain other
ligands, which can thus be regarded as trans-
directing ligands.
• The trans effect which is often called the
kinetic trans effect, refers to the observation
that certain ligands increase the rate of ligand
substitution when positioned trans to the
departing ligand.
7. 7
TRANS DIRECTING
SERIES
• By comparing a large number of reaction rates
the trans directing series can be setup.
• Langford and Gray employed the following
reaction to compare the trans influence of L
and L’.
8. 8
TRANS DIRECTING
SERIES
• They obtained the following trans-directing
series
CN- -
>CH3
-
2)2 2
3 >C6H5 >SO3H >NO2
- -
>
Br-
>Cl-
>py > RNH2 3 >OH-
>aq.
• The trans-directing series might depend upon
the metal complex and also on the incoming
group Y.
9. 9
THEORIES OF TRANS
EFFECT
• Two general theories of trans
effect are currently held :
1. The polarization theory
1. pi-Bonding theory
10. 10
POLARIZATION
THEORY
• It was given by A.A
GRINBERG(1935).
• It is electrostatic in origin.
• According to this theory, the Ligand,
by electrostatic effect, weakens the
bond trans to it and facilitates
substitution in that position.
12. 12
PtX4 TYPE COMPLEX
• When the complex contains four
identical ligands as in PtX4
each polarized by the metal ion to
the same extent.
no induced dipole result in on the
central metal ion.
Thus, none of the four ligands show
trans effect.
13. 13
PtX4 TYPE COMPLEX
• X: same ligands
• Metal-ligand bond length is same due
to absence of polarization.
14. 14
PtLX3 TYPE COMPLEXES
• When complex contains three
identical and one more polarizing
ligand than induced dipole on the
central metal atom results
The electron density to be distorted
via a δ-bond towards the trans
position
The metal-X bond lengthened
16. 16
EXPERIMENTAL
EVIDENCES
• The length of Pt-X bond should
increase with increase in trans-
effect of L.
This fact has been verified
experimentally in complexes, of type
[Pt(NH3)X3]-
(where X=Cl-
,Br-
).
In these complexes, the Pt-X bonds
trans to X-
is longer than the Pt-X
18. 18
EXPERIMENTAL
EVIDENCES
Hence the ligand opposite to the
longer bond will show greater trans-
effect i.e the trans-effect order is X-
NH˃ 3.
Similarly, the Pt-X bond trans to C2H4
in complexes of [Pt(C2H4)X3]-
type
(where X= Cl-
,Br-
has been found
longer than cis to it
20. 20
SUCCESS OF
POLARIZATION THEORY
• Although subject to some criticism,
The polarization theory appears to
account nicely for the behaviour of
ligands like hydride,methide,phenyl
and Chloride.
• In these ligands pi-bonding is
expected to be of minor importance.
21. 21
Weakness
• The emphasis on the trans bond limits
the application of Polarization theory to
the trans influence(it is a
thermodynamic factor which facilitates
substitution of ligand) only.
• It also fails in explaining the high trans
effect of the bonding ligands like
PR3,NO,CO.C2H4 which lie at the high end
of the series.
22. 22
Pi-BONDING THEORY
• This theory was given by Chat in
1955 and Orgel in 1956.
• According to this theory vacant pi
ans pi* orbitals of the metal (pyz and
pxz ) to form M-L pi bond.
• In these cases a reduction of
electron density on the metal , as a
result of back donation donation
from M L.→
23. 23
Pi-BONDING THEORY
• In case of Pt(II) square-planar
complex, PtX2L (L is the pi- bonding
ligand) the dyz orbital of Pt(II) with
a pair of electrons overlaps with the
empty pz orbital of the pi-bonding
ligand to form pi-bond.
• The formation of pi- bond in the
complex increases the electron
density in the direction of L and
diminishes it in the direction of the
ligand, X trans to L.
24. 24
Pi-BONDING THEORY
• The weakening of Pt-X bond trans to
L fascilitates the approach of the
entering ligand.
• Ligands which show pi-bonding trans
effect are PR3,NO,CO,C2H4.
Ligands which follow pi-bonding
theory have more difference in
energy(activation energy) between
ground and transition state.
25. 25
ILLUSTRATION OF
TRANS EFFECT
• The classic example of Trans effect is the
synthesis of cis-platin, cis-
diamminedichloridoplatinum(II). It is
prepared by substituting the two chloro
groups of PtCl4
2-
by ammonia molecules.
• In the first step, any of the chloro group is
substituted by ammonia randomly. But in
the second step, the ammonia group
preferentially substitutes the chloro group
cis to the first ammonia.
26. 26
ILLUSTRATION OF
TRANS EFFECT
• This can be attributed to the fact
that the Cl-
has a larger trans effect
than NH3.
• The second NH3 is added trans to a
Cl−
and therefore cis to the first
NH3.
27. 27
ILLUSTRATION OF
TRANS EFFECT
• Whereas, the trans product is
obtained by starting from Pt(NH3)4
2+
.
• In this case the second Cl group is
substituted preferentially at trans
position to the first one.
28. 28
REFERENCES
• Coe, B. J.; Glenwright, S. J. Trans-effects in
octahedral transition metal
complexes. Coordination Chemistry
Reviews 2000, 203, 5-80.
• Kauffmann, G. B. I'lya I'lich Chernyaev (1893-
1966) and the Trans Effect. J. Chem.
Educ. 1977, 54, 86-89
• Chernyaev, I. I. The mononitrites of bivalent
platinum. I. Ann. inst. platine (USSR) 1926, 4,
243-275
• D.Benson Mechanism of inorganic reactions in
solution
• R.K Sharma Inorganic reaction mechanism
