(a) In `PCl_5`, oxidation state of `P` is `+5`. Phosphorous can exhibit a maximum oxidation state of `+5`. Since `P` cannot have oxidation state greater than `+5`. Therefore, it cannot get oxidised and thus `PCl_5` cannot act as reducing agent. However, `P` can have lower oxidation state as compared to `+5`, and thus can get reduced and `PCl_5` behave as oxidising agent.
(b) Due to large size of iodine, phosphorous cannot accommodate five iodine atom around it, and hence `PI_5` does not exist. Moreover `EN` of iodine is too law to excite `3 s^2` electron to `3 d` orbitals for the formation of five bonds.
