Zn, Cd and Hg have very low melting points due to non involvement of ${\mathrm{d}}^{10}$ configuration in metallic bonding.

Transition metals have close-packed structures in which transition metals are held together by strong metallic bonds which have notable covalent character. For Zn, Cd and Hg the d orbitals are completely filled and no metallic bonds are formed.

Hence, transition metals have low melting and boiling points.