Why chelate compound is more stable

Chelate ligands form more stable complexes than monodendate ligands.The major reason for this is that 'chelate effect is entropy-driven'.It can be explained as follows:

Consider two equilibriums:

• [Co(H2O)6]^2+ + 6NH3 -----> [Co(NH3)6]^3+ + 6H2O                   

 

• [Co(H2O)6]^2+ + 3en-----> [Co(en)3]^2+ + 6H2O

In the first case, ∆S= 0(same no: of molecules on either side)

In the second case, ∆S=+ve ( 4 molecules give (6+1)=7 molecules).

Electronically NH3 and en are the same:

1. Both bind through N atoms.
2. Lewis base strengths are similar.
3. Six Co-N bonds formed in each case.

As a result,∆H values of both the reactions are almost the same. But if you look at their equilibrium constants' values, the 2nd equilibrium has a K value which is about 10^5 times than that of the 1st one.
This is because of the entropy change.
( rem: According to the Eyring equation; ∆G° = -RTlnK = ∆H- T∆S. So as ∆S increases, K value increases since ∆H value are almost the same).

Thus chelate effect is entropy driven. More the no. of binding groups a ligand has; more +ve ∆S is and thus higher the value of Kf will be.