The chief factor responsible for the stability of the chelate ring is the entropy change which can be viewed by probability factor. As per probability factor when monodentate ligands are substituted by the bidentate ligands entropy changes is positive. Stability constants of the complexes are indicated by the terms K stab For the given complexes; K stab values are given as follows
Complex logK stab
Fe(NH 3 ) 6 2+ 4
Co(NH 3 ) 6 2+ 6
Fe(en) 3 2+ 8
en is ethylene diamine
(i) What is the log K stap of Co(en) 3 2+ -
Text Solution
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(i)
Sol. As per Irving-williams order Co 2+ form more stable complexes than Fe 2+ . Bidentate ligand form more stable complex than monodentate ligand.
(ii)
Sol. Co(NH 3 ) 6 2+ + 3 en 
Co(en) 3 2+ + 6 NH 3
Total 4 mole total 7 mol
Since in the substitution reaction the no. of moles increases, hence Δ rS° is positive. Since in both the ligands the donor atoms are only nitrogen, therefore, Co-N bond strength in both the complexes are same.
(iii)
Sol. [Co(NH 3 ) 6 ] 2+ + 3en 
[Co(en) 3 ] 2+ + 6NH 3
K eq = 
K eq = 
∴ log K eq = 8
∴ Δ r G° = –RT ln K eq
= – 2.303 × 8.314 × 10 –3 × log K eq × 300
Δ r G° = –2.303 × 300 × 8.314 × 10 –3 × 8 = – 45.93 KJ 
– 46 KJ
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