Why are trihalides of elements of group 15 less covalent than pentahalides ?

Explanation

(i) In trihalides of elements of group 15, the oxidation state is `+3` while is pentahalides it is `+5`.
(ii) The energy required to removes all the five valence electrons is very large, hence stability of `+5` oxidation state is less and tendency for covalent character is more than `+3` oxidation state.
(iii) Due to higher positive oxidation state `(+5)` of a central atom, they posses larger polarising power and can polarise halides ion `X^(-)` to a greater extent than in the lower `+3` state in their halides.
(iv) Since larger the polarisation, more is the covalent character, therefore trihalides are less covalent than pentahalides.