Why do many transition metals have high melting and boiling points?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — transition metal Modern version of the periodic table of the elements (printable).

Transition metals exhibit high melting and boiling points primarily due to the strength of their metallic bonding, which is significantly more robust than that found in simple metals.12

While simple metals rely on relatively weak bonds formed by sp-electrons, transition metals are characterized by atoms with partially filled d-shells. The d-orbitals are more tightly bound to the ion than sp-orbitals and form covalent-like bonds with the d-orbitals of neighboring atoms. Because these d-bonds are much stronger than the weak sp-bonds, transition metals possess much larger cohesive energy, which is the energy required to hold the crystalline structure together. This increased cohesive energy directly results in the high temperatures required to melt or boil these substances.12