Coup de grâce is a full-round action that allows an attacker to attempt a killing blow against a helpless opponent. A coup de grâce can be administered with a melee weapon, or with a bow or crossbow if the attacker is adjacent to the opponent. An attacker delivering a coup de grâce automatically scores a critical hit, after which the defender must make a Fortitude save (DC 10 + damage dealt) or die. Rogues also gain their extra sneak attack damage for this attack. Delivering a coup de grâce provokes attacks of opportunity from threatening foes. A coup de grâce is not possible against a creature immune to critical hits.
Any opponent which is asleep and has less than four Hit Die is subject to a coup de grâce.
Note: You don't need to be adjacent for a ranged Coup de Grâce to take place. Maybe the distance you need to be is similar to the one of Point_Blank_Shot or Sneak Attack.