The reaction of $CuSO_4$ with excess $KCN$ solution leads to the formation of:

Which of the following species exhibits both $LMCT$ and paramagnetism?

$FeSO_4$ solution mixed with $(NH_4)_2SO_4$ solution in $1:1$ molar ratio gives the test of $Fe^{2+}$ ion,but $CuSO_4$ solution mixed with aqueous ammonia in $1:4$ molar ratio does not give the test of $Cu^{2+}$ ion. Explain why?

$PbCl_{2}$ is insoluble in cold water. Addition of $HCl$ increases its solubility due to

Assertion : Low spin complexes have lesser number of unpaired electrons.
Reason : $[FeF_6]^{3-}$ is a low spin complex.

The most stable ion is