Trans effect And Its Applications

Ms. Nirupama Mahanta
Nutrition and Dietetics
St. Francis Women's College, Hyderabad.

 The Trans effect was first recognized by Ilya Ilich Chernyaev, a russian chemist in
square planar complexes of Platinum(II).
Let us consider the action of NH3 on square planar complex , [Pt(NO)2Cl3]2- to form
[Pt(NO)2Cl2 NH3]- .Theoretically there are two possible reaction products
Experiments have however shown that it is only trans isomer that is formed by the
replacement of Cl- lying trans to NO2
- in [Pt(NO)2Cl3]2- by NH3
The formation of this trans isomer is explained by saying that Cl- ions lying trans to
NO2
- in [Pt(NO)2Cl3]2- is replaced more easily by NH3 than 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
What is trans effect?

The groups like NO2 which control the entering of the ligand to occupy the position
trans to them are known as trans directing groups
Trans effect of a group coordinated to a metal ion is the tendency of that group to
direct an incoming group to occupy the position trans to that group
Trans effect is also defined as the effect of a coordinated group on the rate of the
replacement of a group lying trans to it in a metal complex, eg., in the ligand X which is
trans to L is rapidly replaced by another group such as ‘Y’ to give MLX2Y, L is said to
have larger trans effect or trans directing character. Thus L has greater trans effect than
X(L>X)
By measuring rates of various reactions, a series of ligands can be put
into an order of decreasing trans effect.
The approximate order of decreasing trans effect of some common
ligands is:
CN- , CO , C2H4 , NO > PR3 , H- >CH- > C6H5
- , NO2
- , I- , SCN- (s bonded) > Br-
, Cl- >py , NH3 , OH- , H20
This given series is called trans effect series.
trans-
MLX3
MLX2Y

The ligands lying on the high end of the series have vacant pi orbitals which can accept
electrons from metal orbital to form metal-ligand pi bond. These ligands are, therefore,
called pi bonding ligands. The trans directing ability of these pi bonding ligands
increases with the increase of their ability to form metal-ligand pi bonds.
The trans effect of the ligands which are not able to form metal-ligand pi bonds
increases with the increase of their polarisability , eg.,
polarisability increases
------------------------------------------------------
Cl- < Br- < I-
trans effect increases
------------------------------------------------------
ILLUSTRATIVE EXAMPLE:
Let us consider the preparation of cis and trans isomers of [Pt(C2H4)(NH3)Cl2]0 which is
a square planar complex of Pt(II)
(a) cis-isomer = this isomer can be prepared as follows :
(i)+NH3
----------------
-Cl-
(ii)+C2H4
---------------
-Cl-
[PtCl4]2-
(A)
(B) [Pt(C2H4)(NH3)Cl2]
(C)

In step (i), any of the four Cl- ions present in (A) can be replaced by NH3 molecule to
form (B) . Now in step (ii) , since Cl- ion in (B) has greater trans effect than NH3 molecule
, Cl(2) atom which is trans to Cl(1) in (B) is replaced by C2H4 molecule
(b) trans-isomer = This isomer can be obtained by reversing the addition of NH3 and
C2H4 TO [PtCl4]2- as shown below
(i)+C2H4
---------------
-Cl-
(ii)+NH3
----------------
-Cl-
[PtCl4]2- (A) (B) [Pt(C2H4)(NH3)Cl2]
(C)
In step(i) any of the four Cl- atoms present in (A) can be replaced by C2H4 molecule.
In step (ii) since C2H4 has greater trans effect than Cl- ion , Cl atom trans to C2H4 in
(B) will be replaced by NH3 to form (V) which is a trans isomer.

THEORIES FOR TRANS EFFECT
The trans effect is a kinetic phenomenon affecting the magnitude of activation energy
of a reaction. The stability of both the ground state ( ie.,square planar complex
before the substitution) and the activated complex can, affect the activation energy
required for the substitution reaction.Therefore,any factor that changes the stability
of ground state and /or which changes the stability of activated complex would be a
contributor to the trans effect shown by the attached ligand.There are two theories of
trans of which one relates to the ground state and the other relates to the activated
complex.
1.THE POLARIZATION THEORY:
This theory is primarily concerned with the effects on the ground state.
Let us consider two bonds L-M and M-X trans to each other in a square planar
complex. Suppose the ligand L is more polarizable than the ligand X.The primary
charge on the metal ion polarizes the electron charge cloud on L and thus induces
a dipole in L.The dipole in L,in turn, induces a dipole in M,as shown
+ L
-
+ M
-
-
X
+

The orientation of this dipole on the metal ion is such that it repels the negative
charge in the ligand X which is trans to L.Hence ,the ligand X would be less
attracted by the metal ion because of the presence of L .
According to this theory, the polarization of ligand should be directly related to its
trans effect. This is largely true for ligands which do not form pi bond with the
metal ion.

2.THE Pi BONDING THEORY
This theory satisfactorily explains the trans effect of those attached ligands which
are pi acceptors or pi acids like phosphine,CO,olefins,etc.
 Consider a square planar complex in which an attached ligand L is a pi acid whose
trans effect we want to investigate. The ligand X is situated trans to L.
According to pi bonding theory ,there is a correlation between the tendency of a
ligand to remove the d orbital electron from the metal ion by the pi bonding and the
magnitude of their trans effect. The transition state in the substitution reaction of a
square planar complex is proposed to have a trigonal bipyramidal geometry.

The trigonal bipyramidal transition state can be stabilized if the empty pi orbital of
the ligand L overlaps with a non- bonding filled dπ orbital of the metal to form a pi
bond.
 In fig, the square planar is shown to be in XY plane and the pi bonding is shown
between the empty pi bonding orbital ( say , d orbital)of the ligand L and dxz orbital
of the metal.
It is clear from the figure that electrons from dxz orbitals are withdrawn away from
the incoming nucleophile Y during M-L pi bonding thus stabilizing trigonal
bipyramidal transition state.
Although the pi bonding theory proposes stabilization of the trigonal bipyramidal
state, there is evidence that the M-X bonds are longer when they are placed trans to
the ligand with a strong trans effect than when they are placed cis to such a group
even in the ground state.
Therefore ,a ligand with a strong trans effect, affects the ground state as well as the
transition state. The present view ,therefore , is that both the effects, namely,
polarization which weakens the bond in the ground state and pi bonding which
stabilizes trigonal bipyramidal transition state contribute towards the trans effects
shown by an attached ligand.
There extent of contribution by each depends upon the nature of the ligand.

APPLICATIONS OF TRANS EFFECT :
1. SYNTHETIC APPLICATIONS :-
 Applications of trans effect are shown by the synthesis of many Pt
(II)complexes . For example :-
 (i)Trans –[Pt(R3P)2I2]0 :- can be prepared by the reaction which
consists of the following steps
 In step (i) any of the three ions I- ions present in (A) can be
replaced by R3P . Now in step (ii) since R3P has greater trans
effect than I- ion , it is I1 atom which is trans to R3P in (B) is
replaced by R3P to form trans Pt(R3P)2I2]0

(ii) Diammine dichloroplatinum (II) , [Pt (NH3)2Cl2]0 :- is a square planar
complex of Pt (II). This complex exists in cis and trans isomers (geometrical
isomerism ) .
Preparation of cis- isomer of [Pt (NH3)2Cl2]0 :- The preparation of the
cis isomer is shown in the following figure given below . It may be seen from
this figure that in step (i) any of the four Cl atoms present in (A) can be
replaced by NH3 to form (B) . In step (ii ) , since Cl- ion present in (B) has
greater trans effect than NH3 , Cl2 atom which is trans to Cl1 atom in (B) is
replaced by NH3 molecule to form cis isomer (C ).

Preparation of trans isomer of [Pt (NH3)2Cl2]0 :-
The preparation of the trans isomer is shown in the figure below . From
the figure it may be seen that in step (i) any of the four NH3 molecules in
(A) can be replaced by Cl - atom to form (B). In step (ii) , since Cl- ion
present in (B) has greater trans effect NH3 , 1NH3 molecule which is trans
to Cl - atom in (B) will be replaced by Cl- atom to form the trans isomer
(C) .

(iii) Ammine dichloronitroplatinate(II) [Pt (NH3)2NO2]- :- is a square planar
complex of Pt (II) and exists in cis – and trans – isomers (geometrical
isomers). These isomers can be prepared by the reaction (a) and (b)
respectively.
Reaction (a) :- In step (i) any of the four Cl_ atoms present in (A) can be
replaced by NH3 molecule to form (B ). In step (ii) , since Cl- ion present in
(B) has greater trans effect than NH3 molecule, Cl2 atom which is trans to
Cl1 atom is replaced by NO2
- ion to form cis- isomer of [Pt (NH3)2NO2]- (C ).
Reaction (b) :- In step (i) any of the four Cl- atoms present in (A) can be
replaced by NO2
- ion to form (B). In step (ii) , since NO2
- ion present in (B )
has greater trans effect than Cl- ion , Cl_ atom which is trans to NO2
- will be
replaced by NH3 molecule to form trans- isomer of [Pt (NH3)2NO2]- (C ).

(iv) [Pt (PPh3)(NH3)Cl2)]0 :- is a square planar complex of Pt(II). It exists in
cis and trans –isomers (geometrical isomers ). These isomers can be
prepared by reactions (a) and (b ) respectively.
Reaction (a) :- in step (i)any of the four Cl-atoms in (A) can be replaced by
PPh3 (triphenylphosphine) to form (B). In step (ii), since PPh3 present in (B)
has greater trans directive effect than Cl- ion, Cl-atom which is trans to PPh3
will be replaced by NH3 to form trans – isomer of [Pt (PPh3)(NH3)Cl2)]0 (C ).
Reaction (b) :- in step (i) any of the four Cl-atoms present in (A) can be
replaced by NH3 molecule to form (B). Now in step (ii), since Cl- ion has
greater trans directive effect than NH3 , Cl2 atom which is trans to Cl1 is
replaced by PPh3 to form the cis- isomer ( C ).

2. To Determine Between cis and trans Isomers of
[Pt(NH3)2Cl2] by Kurnakov’s Test : -
Russian chemists have used the concept of trans effect to distinguish
between cis and trans isomers of [Pt(NH3)2Cl2].
It has been done by reacting [Pt(NH3)2Cl2] with thiourea
NH2.CS.NH(tu).
Cis-isomer gives[Pt(tu)4]+2 and trans-isomer gives[Pt(tu)2(NH3)2]
(a) Cl NH3 tu NH3
Pt (i)+2tu Pt
-2Cl tu NH3
Cl NH3

tu NH3 (ii) +2tu tu
tu
Pt
Pt
tu NH3
tu
tu

(b)
H3N Cl H3N Cl
Pt (i)+tu Pt
Cl NH3 -Cl tu NH3
NH3 Cl NH3 tu
(ii)+tu
Pt -Cl Pt
tu NH3 tu NH3

b)In step (i) since Cl atom in (A) has greater trans effect than NH3, Cl
which is trans to CL is replaced by tu to form (B).
In step (ii) ,since tu present in (B) has greater trans effect than Cl atom
,Cl atom trans to ti is replaced by further tu molecule to form
[Pt(NH3)2(tu)2](C).

Trans effect And Its Applications

Trans effect And Its Applications

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Trans effect And Its Applications