Why are pentahalides of $P$,$As$,$Sb$,and $Bi$ more covalent than their trihalides?

(N/A) In pentahalides,the central atom is in the $+5$ oxidation state,whereas in trihalides,it is in the $+3$ oxidation state.
According to Fajan's rule,a higher positive charge on the cation increases its polarizing power.
Since the central atom in pentahalides has a higher positive charge $(+5)$ compared to trihalides $(+3)$,it exerts a stronger polarizing effect on the electron cloud of the halide ions.
This leads to greater sharing of electrons,making pentahalides more covalent than their corresponding trihalides.