Trans Effect

Trans Effect

Trans Effect

The ligand which is already attached to central metal atom has a tendency to direct an incoming ligand to its trans position in ligand displacement reaction in square planar complexes is called trans- effect and such group are labeled as trans directing ligand or trans director or spectator ligand.
Let us consider a substitution reaction in square planar complex, trans MAB3 in which A is a non-leaving group and B trans to A is a leaving group. Since ligand A makes the ligand B labile trans to it so the trans effect is also called labilizing effect. Simply you can say that labilization of ligands trans to other trans directing ligands.
On the basis of quantitative and qualitative observations, trans effect of various ligands are arranged in their decreasing order are given below-
CN > C2H4 > CO > NO > tu > NO2 > SCN > I > Br > Cl > py > NH3 > OH > H2O
Strong trans director ligands (i.e. CN, C2H4, CO, NO etc.) have vacant π and π* orbitals which can accept electron from metal orbitals to form dπ - dπ or dπ - pπ bonds. These ligands are π bonding ligands and strong trans directors. On the other hand, ligands like OH, H2O etc. are very poor trans directors.
A strong trans director ligands has ability to promotes rapid substitution of the ligand trans to itself. Trand effect is a kinetic phenomenon because trans effect of a ligand promotes the rapid substitution of ligand trans to itself while trans influence is a thermodynamic phenomenon. Ligand can influence the ground state propereties of ligands to which they are trans. Good trans directing ligand weaken the bond between the metal and the trans ligand.
trans effect
trans effect
In the above reaction, Second step seems to be against trans-effect rule but it is not so. The trans effect does not predict than any ligand trans to chloride ion should be replaced more radily than any other ligand trans to py but a chloride trans to chloride will be replaced more radily than a chloride trans to py if there is any choice but here there is no any choice in this reaction.

The correct increasing order of trans effect of following species is

a. CN− > Br > C6H5 > NH3
b. Br > CN > NH3 > C6H5
c. NH3 > Br > C6H5 > CN
d. CN > C6H5 > Br > NH3