Question

(a) What do you understand by chelate effect? llustrate your answer with an (8 marks) (b) For the following equilibrium reactions, predict whether the equilibrium example. constant (K) will be greater or smaller than 1. Briefly explain. (4 marks)

Answer 1

[a]

Chelate effect: The enhanced affinity of some ligands [chelating ligands] towards the metal ion compared to non-chelating ligands is called chelating effect. This is observed because the ligands can readily form bonds with the metal ion by donating their readily available lone pair into the empty orbitals of the metal ion to complete their octet, as seen in coordination compounds. One or more than one lone pair can be shared between the metal ion and the ligand, in which case it may be called monodentate, bidentate and so on. eg hexaamminecobalt (III) chloride

H2N NH 2

[b]

The spectrochemical series of ligands is as follows [incomplete list]:

O₂^2?< I^? < Br^? < S^2? < SCN^? (S–bonded) < Cl^? < N₃^? < F^?< NCO^? < OH^? < C₂O₄^2? ? H₂O < NCS^? (N–bonded) < CH₃CN < py (pyridine) < NH₃ < en (ethylenediamine) < bipy (2,2'-bipyridine) < phen (1,10-phenanthroline) < NO₂^? < PPh₃ < CN^? < CO

The leftmost ligands are the weakest and those on the right side are the strongest

[i] Following the spectrochemical series, CN^- is a stronger ligand than Cl^- and chelation by CN^- would form a more stable compound. Since the rate of backward reaction will be more, equilibrium const K would be smaller than 1.

[ii] In the second reaction, the empty orbital of the phosphorous atom is bonded with a BF3 molecule, which will further help in delocalization of electrons between the phosphorous and boron atoms, which are donated by the three phenyl groups, owing to higher electronegativity of F than that of Br, and thus make the BF3 bonded molecule more stable. Thus equilibrium constant K would be less than 1.