This question has two components: firstly, can a character with Assassin levels trigger their Death Attack (after the three rounds of study, of course) by making a Coup De Grace against the helpless opponent they studied? Secondly, can I make a coup de grace and/or death attack with a melee touch spell such as Shocking Grasp, Vampiric Touch or Poison?
Let's check the definitions.
Death attack (emphasis mine):
If an assassin studies his victim for 3 rounds and then makes a sneak attack with a melee weapon that successfully deals damage.
As a full-round action, you can use a melee weapon to deliver a coup de grace to a helpless opponent. You can also use a bow or crossbow, provided you are adjacent to the target.
- Yes, assassin does trigger Death Attack when makes coup de grace, provided he makes it with a melee weapon.
- RAW, no. Complete Arcane (pp 85-86) tells us nothing about weaponlike spells and death attacks/coup de graces, and it features a list of things you can do with weaponlike spell. Wouldn't hurt to houserule otherwise, since you can sneak attack or critical hit with a weaponlike spell. FAQ agrees on this matter, albeit it mentions rays only:
Can you perform a coup de grace with a ray spell?
No. The coup de grace action specifically allows you to use a bow or crossbow, provided you are adjacent to the target (PH 154).
That said, allowing spells with a ranged touch attack to deliver a coup de grace against an adjacent target wouldn’t be an unreasonable house rule.
Complete Mage, Spell Compendium and Rules Compendium have no additional information on the subject.
Spellwarp Sniper 5 gains the ability to coup the grace with a ray spell that deals hit point damage.
Ray Mastery You can deliver a coup de grace with a ray spell that deals hit point damage .You must be adjacent to your target to deliver the coup de grace .
This ability still doesn't allow to trigger Death Attack.
Addressing the second part, I'd say that yes, you can coup de grace with a touch spell. I'm basing this off the following, from Combat Actions:
Touch Attacks: Touching an opponent with a touch spell is considered to be an armed attack and therefore does not provoke attacks of opportunity. However, the act of casting a spell does provoke an attack of opportunity. Touch attacks come in two types: melee touch attacks and ranged touch attacks. You can score critical hits with either type of attack.
So, an attack with a touch spell is considered a melee attack, and is considered an armed attack. An armed attack is generally one made with a weapon. Coup de grace requires you use "a melee weapon". This strongly points towards being able to use a touch attack for a coup de grace.
Further, you can score critical hits and sneak attack damage on this type of attack, and can deliver touch spells with a punch, so every ounce of "in game" sense tells us you should be able to deliver a coup de grace this way.
One thing you didn't bring up: In most cases you'd have to have touch spell already prepped, so you could simply deliver it with a regular unarmed attack -- which might bypass some of this discussion.
Note on the RAW
I think the wording of coup de grace would technically let you use a dagger to perform a coup de grace from across the room. They really meant to restrict it to a "melee attack" rather than a "melee weapon", and if they'd worded it differently this whole question would have been easier to answer. :)
I would always answer this question "Yes".
According to the 3.5 SRD "Touching an opponent with a touch spell is considered to be an armed attack", and it certainly is melee.
Since a touch spell that causes damage is basically the same as a natural weapon, or a trained unarmed strike, both of which can be used to coup de grace, I'm not sure why the answer would ever be "no". If an unarmed strike can coup de grace, Shocking Grasp can certainly coup de grace.
When the RAW aren't abundantly clear, I tend to fall back on visualization and my own sense of how things work in the world I'm running. The sections quoted by other answers here can certainly be interpreted as implied. However, when I envision a coup de grace with a dagger, I can understand how anyone could work out how to inflict a mortal wound against a helpless foe. That's harder to envision when using shocking grasp, chill touch, etc. without some serious anatomy knowledge. I would probably find my myself ruling on a case-by-case basis based on the spell effects and the character's skills.