Which of the following complexes formed by `Cu^(2+)` ions is most stable ?

Options

A. `Cu^(2+)+4NH_(3) rarr [Cu(NH_(3))_(4)]^(2+), log K=11.6`
B. `Cu^(2+)+4CN^(-) rarr [Cu(CN)_(4)]^(2-), log K=27.3`
C. `Cu^(2+)+2en^(-) rarr [Cu(en)_(2)]^(2+), log K=15.4`
D. `Cu^(2+)+4H_(2)O rarr [Cu(H_(2)O)_(4)]^(2+), log K =8.9`

Answer

B

Explanation

Greater the value of log K, greater will be stability of complex compound formed. For reaction,
`Cu^(2+) + 4CN^(-) rarr [Cu(CN)_(4)]^(2-)`
`K=([(Cu(CN)_(4))^(2-)])/([Cu^(2+)][CN^(-)]^(4))` and `log K=27.3`
For this reaction, log K has highest value among the given four reactions. Hence, K will also be higher among these four. i.e., stability of the complexes will be highest among these four complexes.