Ammonia is a Lewis base. It forms complexes with cations. Which one of the following cations does not form a complex with ammonia?

The stepwise formation of $[Cu(NH_{3})_{4}]^{2+}$ is given below:
$Cu^{2+} + NH_{3} \rightleftharpoons [Cu(NH_{3})]^{2+} \quad K_{1}$
$[Cu(NH_{3})]^{2+} + NH_{3} \rightleftharpoons [Cu(NH_{3})_{2}]^{2+} \quad K_{2}$
$[Cu(NH_{3})_{2}]^{2+} + NH_{3} \rightleftharpoons [Cu(NH_{3})_{3}]^{2+} \quad K_{3}$
$[Cu(NH_{3})_{3}]^{2+} + NH_{3} \rightleftharpoons [Cu(NH_{3})_{4}]^{2+} \quad K_{4}$
The values of stability constants $K_{1}, K_{2}, K_{3}$ and $K_{4}$ are $10^{4}, 1.58 \times 10^{3}, 5 \times 10^{2}$ and $10^{2}$ respectively. The overall equilibrium constant for dissociation of $[Cu(NH_{3})_{4}]^{2+}$ is $x \times 10^{-12}$. The value of $x$ is ...............
(Rounded off to the nearest integer)

In the isoelectronic series of metal carbonyls,the $CO$ bond strength is expected to increase in the order:

Solubility of iodine in water is greatly increased by the addition of iodide ions because of the formation of ......

Aluminium trifluoride is insoluble in anhydrous $HF$ but dissolves on addition of $NaF$. Aluminium trifluoride precipitates out of the resulting solution when gaseous $BF_3$ is bubbled through. Give reasons.

On adding an excess of $NH_3$ solution to $CuSO_4$ solution,the dark blue colour is due to the formation of which complex?