Question

Why are pentahalides more covalent than trihalides?

Answer 1

The oxidation state of +5 in pentahalides is more as compared to +3 oxidation state in trihalides. Due to the higher positive oxidation state of central atom in pentahalide state, these atoms will have larger polarizing power than the halogen atom attached to them.
But In case of trihalides due to +3 oxidation state the central atom will polarize the halogen atom to a lesser extent as compared to pentahalide state.
Hence due to larger polarization of bond in pentahalide state as compared to trihalide state, the pentahalides are more covalent than trihalides.