Why are pentahalides more covalent than trihalides ?

Pentahalides (like MX₅) means the compounds in which metal is bonded with five halogen atoms. Thus, the oxidation state of metal here is +5. Similarly, trihalides (like MX₃) means the compounds in which metal is bonded with three halogen atoms. Thus, the oxidation state of metal here is +3. Now, as the polarizing power is directly proportional to the charge, the metals with the higher charge will have higher polarizing power. Hence, Pentahalides are more covalent than trihalides.

Note: Polarizing power is the ability of a cation to distort an anion.