Trans effect And Its Applications

Nutrition and Dietetics

1. TRANS EFFECT AND ITS APPLICATIONS Ms. Nirupama Mahanta Nutrition and Dietetics St. Francis Women's College, Hyderabad.

2.   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 NH 3 on square planar complex , [Pt(NO) 2 Cl 3 ] 2- to form [Pt(NO) 2 Cl 2 NH 3 ] - .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 NO 2 - in [Pt(NO) 2 Cl 3 ] 2- by NH 3 The formation of this trans isomer is explained by saying that Cl - ions lying trans to NO 2 - in [Pt(NO) 2 Cl 3 ] 2- is replaced more easily by NH 3 than either of the two Cl - ions which are lying cis to NO 2 - ion. The phenomenon of such type of replacement is known as trans effect What is trans effect?

3. The groups like NO 2 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 MLX 2 Y, 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 , C 2 H 4 , NO > PR 3 , H - >CH - > C 6 H 5 - , NO 2 - , I - , SCN - (s bonded) > Br - , Cl - >py , NH 3 , OH - , H 2 This given series is called trans effect series . trans- MLX 3 MLX 2 Y

4. 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(C 2 H 4 )(NH 3 )Cl 2 ] which is a square planar complex of Pt(II) (a) cis-isomer = this isomer can be prepared as follows : (i)+NH 3 ----------------  -Cl- (ii)+C 2 H 4 ---------------  -Cl- [PtCl 4 ] 2- (A) (B) [Pt(C 2 H 4 )(NH 3 )Cl 2 ] (C)

5. In step (i), any of the four Cl - ions present in (A) can be replaced by NH 3 molecule to form (B) . Now in step (ii) , since Cl - ion in (B) has greater trans effect than NH 3 molecule , Cl(2) atom which is trans to Cl(1) in (B) is replaced by C 2 H 4 molecule (b) trans-isomer = This isomer can be obtained by reversing the addition of NH3 and C2H4 TO [PtCl4]2- as shown below (i)+C 2 H 4 ---------------  -Cl - (ii)+NH 3 ----------------  -Cl - [PtCl 4 ] 2- (A) (B) [Pt(C 2 H 4 )(NH 3 )Cl 2 ] (C) In step(i) any of the four Cl - atoms present in (A) can be replaced by C 2 H 4 molecule. In step (ii) since C 2 H 4 has greater trans effect than Cl - ion , Cl atom trans to C2H4 in (B) will be replaced by NH 3 to form (V) which is a trans isomer.

6. 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 +

7. 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.  

8. 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.

9. 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.

10. 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(R 3 P) 2 I 2 ] :- 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 R 3 P . Now in step (ii) since R 3 P has greater trans effect than I - ion , it is I 1 atom which is trans to R 3 P in (B) is replaced by R 3 P to form trans Pt(R 3 P) 2 I 2 ]

11. (ii) Diammine dichloroplatinum (II) , [Pt (NH 3 ) 2 Cl 2 ] :- is a square planar complex of Pt (II). This complex exists in cis and trans isomers (geometrical isomerism ) . Preparation of cis- isomer of [Pt (NH 3 ) 2 Cl 2 ] :- 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 NH 3 to form (B) . In step (ii ) , since Cl - ion present in (B) has greater trans effect than NH 3 , Cl 2 atom which is trans to Cl 1 atom in (B) is replaced by NH 3 molecule to form cis isomer (C ).

12. Preparation of trans isomer of [Pt (NH 3 ) 2 Cl 2 ] :- 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 NH 3 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 NH 3 , 1 NH 3 molecule which is trans to Cl - atom in (B) will be replaced by Cl- atom to form the trans isomer (C) .

13. (iii) Ammine dichloronitroplatinate(II) [Pt (NH 3 ) 2 NO 2 ] - :- 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 NH 3 molecule to form (B ). In step (ii) , since Cl - ion present in (B) has greater trans effect than NH 3 molecule, Cl 2 atom which is trans to Cl 1 atom is replaced by NO 2 - ion to form cis- isomer of [Pt (NH 3 ) 2 NO 2 ] - (C ). Reaction (b) :- In step (i) any of the four Cl - atoms present in (A) can be replaced by NO 2 - ion to form (B). In step (ii) , since NO 2 - ion present in (B ) has greater trans effect than Cl - ion , Cl _ atom which is trans to NO 2 - will be replaced by NH 3 molecule to form trans- isomer of [Pt (NH 3 ) 2 NO 2 ] - (C ).

14. (iv) [Pt (PPh 3 )(NH 3 )Cl 2 )] :- 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 PPh 3 (triphenylphosphine) to form (B). In step (ii), since PPh 3 present in (B) has greater trans directive effect than Cl - ion, Cl-atom which is trans to PPh 3 will be replaced by NH 3 to form trans – isomer of [Pt (PPh 3 )(NH 3 )Cl 2 )] (C ). Reaction (b) :- in step (i) any of the four Cl-atoms present in (A) can be replaced by NH 3 molecule to form (B). Now in step (ii), since Cl - ion has greater trans directive effect than NH 3 , Cl 2 atom which is trans to Cl 1 is replaced by PPh 3 to form the cis- isomer ( C ).

15. 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

16. tu NH3 (ii) +2tu tu tu Pt Pt tu NH3 tu tu (A)In step (i) both Cl-atoms present in cis-isomer of [Pt(NH3)2Cl2](A) are replaced by two tu molecules to form (B). In step (ii),since trans effect of tu is greater than that of NH3 molecules which are trans to tu in (B) are replaced by two tu molecules to form [Pt(tu)4](C).

17. (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

18. 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).

19. THANK YOU