Combat Modifiers
I understand that if an opponent is grappled and pinned
that opponent is not helpless and therefore not subject to a
coup de grace. But if that same opponent was rendered
unconscious, could you perform a coup de grace as an
unarmed attack? If so, what would the damage be?
You are correct, pinned characters are not helpless and are
not subject to coup de grace. You can use an unarmed attack
for a coup de grace. The target has to save as noted on page 133
of the Player™s Handbook. Note that you can deal normal
damage with an unarmed attack by taking at a -4 penalty on the
attack. (You automatically hit when you deal the coup de grace,
so the penalty isn't relevant.) If you choose to deal real
damage, you’ll eventually kill a helpless foe even if the foe
keeps making successful saving throws.