Which complex compound is most stable?

The most stable ion is

Violet colour of $[Ti(H_2O)_6]Cl_3$ on heating changes to $........$

$A$ first row transition metal $(M)$ does not liberate $H_{2}$ gas from dilute $HCl$. $1 \ mol$ of aqueous solution of $MSO_{4}$ is treated with excess of aqueous $KCN$ and then $H_{2}S_{(g)}$ is passed through the solution. The amount of $MS$ (metal sulphide) formed from the above reaction is . . . . . . $mol$.

What is the coordination entity formed when excess of aqueous $KCN$ is added to an aqueous solution of copper sulphate? Why is it that no precipitate of copper sulphide is obtained when $H_{2}S_{(g)}$ is passed through this solution?

An aqueous solution of $FeSO_4$ gives tests for both $Fe^{2+}$ and $SO_4^{2-}$ ions,but after the addition of an excess of $KCN$,the solution ceases to give a test for $Fe^{2+}$. This is due to the formation of: