Halides of group 15 elements
Fajans' Rule is  used to predict whether a chemical bond will be covalent or ionic.

Polarization will be increased by:

1) High charge and small size of the cation

2) Ionic potential 

3) High charge and large size of the anion

4) The polarizability of an anion is related to the deformability of its                                            electron cloud (i.e. its "softness")

5) An incomplete valence shell electron configuration

In pentahalides, the oxidation state is +5 and in trihalides, the oxidation state is +3. Since the metal ion with a high charge has more polarizing power, pentahalides are more covalent than trihalides.