(a) Transition metals are able to form complex compounds due to the small size of metal, high ionic charge and availability of vacant $\mathit{d}$-orbital.
(b) ${\mathrm{E}}^{\circ }$ values of $\left({\mathrm{Mn}}^{2+}∕\mathrm{Mn}\right)$ and $\left({\mathrm{Zn}}^{2+}∕\mathrm{Zn}\right)$ are more negative than expected due to the greater stability of half filled $\mathit{d}$-subshell of ${\mathrm{Mn}}^{2+}\left(3{\mathit{d}}^5\right)$ and completely filled $\mathit{d}$-subshell of ${\mathrm{Zn}}^{2+}\left(3{\mathit{d}}^{10}\right)$.
(c) Due to a very small energy gap between $5\mathit{f},6\mathit{d}$ and $7\mathrm{}\mathit{s}$-subshells resulting in easier excitation of the outermost electrons to higher energy levels.