Transition elements in lower oxidation states act as Lewis acid because

The reaction of nitroprusside anion with sulphide ion gives purple colouration due to the formation of

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

Dimethyl glyoxime forms a coloured complex with

Explain the chelating effect or chelation.