This ppt is helpful in understanding different theories of Trans effect in square planar complexes

1. Dr. Geeta Tewari
Department of Chemistry
D. S. B. Campus
Kumaun University, Nainital
Email: geeta_k@rediffmail.com
Substitution Reactions in Square
Planar Complexes: Part 4, Theories of
Trans Effect

2. Trans-effect
Trans effect is the labilization (making more
reactive) of ligands that are trans to certain ligands
(spectator ligands), which can be regarded as trans-
directing ligands.
It is a kinetic phenomenon.
Trans-effect was discovered by Ilya Ilich Chernyaev
(1926).

3. Synthesis of cis- and trans-[Pt(NH3)2(Cl2)]
P
t
H
3
N
H
3
N
N
H
3
N
H
3
+
C
l
-
-
N
H
3
2
+ +
P
t
C
l
H
3
N
N
H
3
P
t
H
3
N
H
3
N
C
l
N
H
3 C
l
+
C
l
-
-
N
H
3
Pt
Cl
Cl
Cl
Cl
+NH3
2-
Pt
Cl
Cl
NH3
Cl
-
Pt
Cl
Cl
NH3
NH3
+NH3
-Cl-
-Cl-
Cis-[Pt(NH3)2(Cl2)]; cis-platin
Trans[Pt(NH3)2(Cl2)]; trans-platin

4. • The theory was given by Grinberg (1927).
• Two factors are important in explaining trans-effect.
(1) Pt-X (leaving ligand) bond weakening.
(2) Stabilization of trigonal bipyramidal transition
state.
• The trans-directing ability of a ligand is correlated
with its polarizability.
Polarization Theory

5.  PtX4 complex
 Pt (II) induces dipole in all ligands with equal
amount.
 Resultant dipole is zero.
 No trans effect is shown by any ligand.
Polarization Theory
P
t
(
I
I
)
-
+
-
+
-
+
-
+
X
X
X
X

6. PtX3L Complex
L is larger, hence more polarizability than X
trans to it.
Primary charge on Pt(II) induces dipole in L
and in turn L also induces dipole in Pt(II). This
causes polarization of both (Pt2+ and L).
This will reduce the positive charge at the
point of Pt(II) towards the ligand (X) trans to L
and induce negative charge at the point of Pt(II)
which is towards L.
The induced negative charge will repel X and
Pt(II)-X bond length lengthen and weakened.
Polarization Theory
P
t
(
I
I
)
-
+
- +
-
+
-
+
X
X
X
L
-
-
-
-
+
+
+
+
P
t
(
I
I
)
-
+
-
+
-
+
X
X
-
X
L
-
-
-
-
+
+
+
+
+
+
+
+
+
-
-
-
-
-

7.  M-X bond weakening increases the energy of ground state of
the reacting complex.
 Thus, decreases the activation energy and increases the rate
of substitution reaction.
Polarization Theory
Reactant
Product
Transition state
Reactant
Product
Transition state
Ea
Ea
P
t
A
A
L X

8.  Trans-effect of L ligand weakens the M-X bond
in [Pt(A)2LX].
 This weakening of bond (M-X) increases the
energy of the ground state of metal complex.
 Thus, activation energy will decrease and rate of
the substitution reaction will increase.
 This theory explains the trans effect of σ-donor
ligands (I-, Cl-, OH-, H2O, NH3).
Polarization Theory

9. π-Bonding Theory
 The theory explains the trans-effect of π-acceptor ligands (CO,
CN-, NO, C2H4).
 The filled metal orbitals provide electrons to the empty π*
orbitals of these π-acceptor ligands .
 This bonding stabilizes the transition state or intermediate.
 The donation of electrons towards L (C2H4- a π-acceptor
ligand) from metal cation, decreases the electron density
towards X ligand (trans to L).
 This causes weakening of M-X bond and entrance of the
incoming ligand (Y) become easy (high rate of substitution
reaction).
 This suggests that the transition state (Intermediate-TBP)
stabilizes and thus, rate of substitution will enhance.
Selected Topics in Inorganic Chemistry. 2014. W.H. Malik, G.D. Tuli and R.D. Madan. 8th Edition. S Chand Publisher.

10. π-Bonding Theory
 L=CO; Pt(II) will donate electrons pairs from filled d-
orbitals to the empty π antibonding molecular orbitals of
CO.
 L=PR3; Pt(II) will donate electrons pairs from filled d-
orbitals to the empty σ* orbitals of PR3.
https://snappygoat.com/s/?q=bestof%3AQuintuple+bond+orbital+diagram2.png+en+diagram+of+d-orbital+interactions+involved+in+metal-
metal+quintuple+bonding+own+Jeremyi77+2008-12-11+Hybridized+atomic#,0,114.
R3P M

11. Activation energy and trans-effect
Reactant
Product
Transition state
Ea
Reactant
Product
Transition state
Ea
Inorganic Chemistry. 2005. G.L. Meissler and D.A. Tarr. 3rd Edition. Pearson Education.

12. • During the substitution reaction, trans directing
ligand (L) is present on the equatorial position in
TBP intermediate.
• In this way, L forces the leaving ligand (X) present
at the trans position of L to substitute and form the
square planar product.
Activation energy and trans-effect
L P
t
Y
X