Question

Which of the following complexes formed by $C{u}^{2+}$ ions is most stable?
(a) $C{u}^{2+}+4N{H}_3\to \left[Cu\right(N{H}_3{)}_4{]}^{2+},logK=11.6$
(b) $C{u}^{2+}+4C{N}^{-}\to \left[Cu\right(CN{)}_4{]}^{2-},logK=27.3$
(c) $C{u}^{2+}+2e{n}^{-}\to \left[Cu\right(en{)}_2{]}^{2+},logK=15.4$
(d) $C{u}^{2+}+4H_{2}O\to \left[Cu\right(H_{2}O{)}_4{]}^{2+},logK=8.9$

Answer 1

Answer (b). Greater the value of logK, greater will be the stability of the complex compound formed.
For reaction,
$C{u}^{2+}+4C{N}^{-}\to \left[Cu\right(CN{)}_4{]}^{2-}$
$K=\frac{\left[\right(Cu\left(CN{)}_4{)}^{2-}\right]}{\left[C{u}^{2+}\right][C{N}^{-}{]}^4}andlogK=27.3$
For this reaction, log K has the highest value among the given four reactions. Hence, K will also be higher among these four, i.e., the stability of the complexes will be highest among these four complexes.