The inability of ${\mathrm{ns}}^2$ electrons of the valence shell to participate in bonding on moving down the group in heavier $\mathrm{p}-$block elements is called inert pair effect.

As a result,$\mathrm{Pb}\left(\mathrm{II}\right)$is more stable than $\mathrm{Pb}\left(\mathrm{IV}\right)$

$\mathrm{Sn}\left(\mathrm{IV}\right)$ is more stable than $\mathrm{Sn}\left(\mathrm{II}\right)$

$\therefore$ $\mathrm{Pb}\left(\mathrm{IV}\right)$ is easily reduced to $\mathrm{Pb}\left(\mathrm{II}\right)$.

$\therefore$ $\mathrm{Pb}\left(\mathrm{IV}\right)$ is an oxidizing agent.

$\mathrm{Sn}\left(\mathrm{II}\right)$ is easily oxidized to $\mathrm{Sn}\left(\mathrm{IV}\right)$

$\therefore$ $\mathrm{Sn}\left(\mathrm{II}\right)$is reducing agent.