Can you use activatable items on baron??

UNIQUE: Drains target champion, dealing 10% of the target's maximum health in physical damage (min. 100) and healing you for the damage dealt. Additionally you steal 25% of their movement speed for 3 seconds - 90 second cooldown. (450 range)

Unique: Deals 15% of target champion's maximum health in magic damage and applies the debuff, Doom. Doom amplifies all magic damage that champion takes by 20% for 4 seconds. 90 second cooldown (750 range).

You tell me.

no. purple sword you can

If it's targetable, it most definitely cannot be casted on minions or monsters.

Only item actives I can think of that do affect monsters are Plenty of passives work, though, unless it specifically states champions.

It's a fair enough question... As after all, for the longest while, (like 'til the beginning of Season 3) Ignite was usable on Baron, in spite of the description clearly stating "champion."

Though to be a bit more thorough on the answer, no Active which has a SINGLE target can be used on any neutral monsters. However, four items have AoE actives that CAN hit and affect them: Frost Queen's Claim, Odyn's Veil, Ravenous Hydra, and Randuin's Omen. (though of course, the last of those four deals no damage)

Besides that, Banner of Command's "Promote" is the only effect that can target a MINION (not monster) while Ohmwrecker targets turrets. All other actives are either self-targetted, or specifically target a single enemy champion,