Which of the following complexes formed by Cu++ ions is most stable?
1. Cu2+ + 4NH3Cu(NH3)4+2  logK = 11.6
2. Cu2+ + 4CN-Cu(CN)42-  logK = 27.3
3. Cu2+ + 2en-Cu(en)22+  log K =15.4
4. Cu2+ + 4H2OCu(H2O)42+  logK = 8.9

Hint: Stability of complex directly proportional to the logK value

The greater the value of logK, the greater will be the stability of complex compounds formed

For reaction,

Cu2 + 4CN-Cu(CN)42-K=Cu(CN)42-Cu2+CN-4 and logK=27.3

For this reaction, log K has the highest value among the given four reactions. Hence, K will

also be higher. Hence, it will have a higher stability.