Question

Why is ${\mathrm{PCl}}_5$ more covalent than ${\mathrm{PCl}}_3$?

Answer 1

Polarizability and Fajan's rule

• Fajan's rule is based on the concept of polarization.
• The greater the positive oxidation state of the central metal atom, the greater its polarizing power.
• Hence the covalent nature of the bond produced between the central metal atom and other atoms.
• The polar strength of the cation and the anion's polarizability are the two main parameters that influence the extent of polarization of an anion.

${\mathbf{PCl}}_{\mathbf{5}}$ more covalent than ${\mathbf{PCl}}_{\mathbf{3}}$

• In Phosphorus pentachloride $\left({\mathrm{PCl}}_5\right)$, the central metal atom, $\mathrm{P}$ is in $+5$ oxidation state.
• In Phosphorus trichloride $\left({\mathrm{PCl}}_3\right)$, the central metal atom, $\mathrm{P}$ is in $+3$ oxidation state.
• ${\mathrm{PCl}}_5$ has greater the positive oxidation state than ${\mathrm{PCl}}_3$.
• Therefore, ${\mathrm{PCl}}_5$ is more covalent than ${\mathrm{PCl}}_3$.