Trans effect in coordination chemistry and effects

trans effect in coordination complexes

1. TRANS EFFECT PRESENTED BY: SANKET KUMAR DAS 1 & SACHIN BIDIKA

2. WHAT IS TRANS EFFECT ? In inorganic chemistry, the trans effect is the increased lability of ligands that are trans to certain other ligands . For example consider the following substitution reaction ,where theoretically two reaction products are possible that are given below 2

3. What is trans ligand ? The ligands which increases the lability of the other certain ligand which is trans to it. Role of trans ligand: 1.kinetic role. 2.Thermodynamic role. By measuring the rate 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: This given series is trans effect series . 3

4. Examples of trans effect: 3 4 If, on the other hand, one starts from Pt(NH ) 2+ , the trans product is obtained instead: 4

5. FACTORS EFFECTING THE TRANS EFFECT: 1.LIGAND STRENGTH: Stronger ligands, like those with pi acceptor properties , enhance the rate of substitution of the trans ligand. 2.ELECTRONIC INTERACTION: The electronic properties of the ligands, such as their ability to donate or withdraw electrons impact the trans effect. 3. STERIC EFFECT: Bulky ligands can hinder the approach of incoming ligands ,reducing the substitution rate of the trans ligand . 5

6. THEORIES OF TRANS EFFECT: A . Polarization theory: Let us consider two bonds L-M and M-X trans to each other in a square planner 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 induced a dipole in L. The dipole in L, in turn ,induced a dipole in M, as shown The orientation of the dipole moment on the metal ion is such that it repeals 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. 6

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8. 2.THE Pi BONDING THORY: This theory satisfactorily explain the trans effect of those attached ligands are Pi acceptors or Pi acids like olefine, phosphine, CO, etc. Consider a square planar complex in which an attached ligand L is a Pi acid whose trans effect we want to investigate. The trans ligand X is situated at 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 Pi bonding and the magnitude of their trans effect .The transition state in the substitution reaction of a square planner 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 overlap with a non- bonding filled d-Pi orbital of the metal to form a bond. 8

9. Applications of the trans effect 1. Synthesis of the cis and trans platin: [PtCl2(NH3)2] exists in two isomeric forms as given below. (a) cis-[PtCl2(NH3)2] (b) trans- [PtCl2(NH3)2] These two isomers can be synthesized via a number of number chemical routes by exploiting the trans effect. However, the most common are given below. 9

10. The initial raw materials for the synthesis of isomeric forms are different as the Cl– has a higher trans effect than NH3. Hence, the trans effect plays a major role in product formation. It is worth noting that the cis-platin is having very important therapeutic uses in the treatment of cancer 10

11. Differentiating between cis and trans isomers of [PtCl2(NH3)2]: A Russian scientist Kurnakov has used the trans effect to distinguish between the cis and trans platin. The experimental route he invented is named after him as the Kurnakov test in which thiourea is used as a primary component. The trans effect of thiourea is greater than chloride and amine ligands. The addition of thiourea to the trans platin results in the replacement of trans chloride ions and the reaction stops. However, when the thiourea is added to the cis isomer, all the four ligands are displaced and [Pt(tu)4] 2+ is formed. 11

12. 3.Synthesis of the isomers of [Pt(PR3)2Cl2]: The trans-effect order of the two groups in [Pt(PR3)2Cl2] complex is PR3 > Cl– . (a) (b) Figure 11. Synthesis route for (a) cis-[Pt(PR3)2Cl2] and trans-[Pt(PR3)2Cl2] complexes. 12

13. Summary of Trans Effect : •The trans effect gives the relationship between the reaction rate and the nature of the ligand trans to the leaving group. •The ability of a group to direct substitution into its opposite position in the square. •The trans directing ability of the Pi bonding ligands increases with the increase of the ability to form metal-ligand Pi bond. •The trans effect of the ligands which are not able to form M-L Pi bonds, increases with the increase of their polarisability. •Ligands highest in the series are stronger Pi acceptor, followed by strong Sigma donor. Ligands at the low end of the series have neither strong sigma donor not Pi acceptor abilities. 13

14. Thank you!