The green coloured complex ion of nickel in its aqueous solution is

$A$ pink coloured aqueous solution of $Co(NO_3)_2$ turns blue on the addition of $HCl$ gradually. This colour change happens due to the formation of

The most stable complex among the following is

An aqueous solution of $CoCl_2$ on addition of excess of concentrated $HCl$ turns blue due to the formation of:

The colour of cobalt chloride solution is:

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.