89.
[Cu(CN)4]2+ is more stable complex than [Cu(NH3)4]+. Why?
The oxidation state in the complex [Cu(CN)4]2+ is +2 and oxidation state in the [Cu(NH3)4]+ complex is +1. Stability depends on the hydration energy (enthalpy) of the ions when they bond to the water molecules. The Cu2+ ion has a greater charge density than Cu+ ion and thus forms much stronger bonds releasing more energy. The extra energy needed for the second ionization of the copper is more than compensated for by the hydration, so much so that the Cu+ ion loses an electron to become Cu2+ which can then release this hydration energy.
In aqueous solution Cu+ disproportionate to Cu2+ and Cu
2Cu+------>Cu2+ +Cu.
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