Why Does Carbon Monoxide (CO) Have a Triple Bond?
Carbon monoxide looks like it should not work. Carbon brings 4 outer electrons, oxygen brings 6, and yet the two are held together by a triple bond, the same as in nitrogen, N₂. Where does that third bond come from? The answer is one clever move: a dative (coordinate) bond. Once you see it, CO makes complete sense.
The puzzle
In a normal covalent bond, each atom donates one electron to the shared pair. Carbon has 4 outer electrons to play with, oxygen has 6. If they only shared electrons in the normal way, carbon could reach at most a double bond to oxygen, and it would still be two electrons short of an octet. So a simple “each gives one” picture cannot give a stable molecule. Something else has to happen.
Dot-and-cross, step by step
Total outer electrons to place: 4 (from C) + 6 (from O) = 10. That is the same count as N₂, which is exactly why CO copies nitrogen’s triple bond.
- Form two ordinary covalent bonds between C and O. Carbon puts in two electrons, oxygen puts in two. That uses 2 of carbon’s electrons and 2 of oxygen’s.
- Take stock. Carbon now has 1 lone pair left and only 6 electrons around it (still short). Oxygen has 2 lone pairs left and already has a full octet.
- Make the third bond dative. Oxygen has a spare lone pair and carbon has room, so oxygen donates a whole lone pair to form the third bond. Both electrons of that bond come from oxygen. This is the dative bond, often drawn as an arrow: C≡O with the arrow pointing from O to C.
- Check octets. Carbon: three bonds (6 shared electrons) + 1 lone pair = 8. Oxygen: three bonds (6 shared) + 1 lone pair = 8. Both atoms now have a full octet.
So the structure is :C≡O:, with one lone pair on each atom and three bonds between them. The crucial point is that one of those three bonds is dative, donated entirely by oxygen.
For a clean dot-and-cross you can build and rotate yourself, try the free ChemBank dot-and-cross simulation, where CO is one of the worked 10-electron species.
Formal charges, explained simply
“Formal charge” just asks: compared with a lone atom, has this atom gained or lost a share of electrons in the molecule? Count each atom’s lone-pair electrons, add half of its bonding electrons, then compare with its normal number of outer electrons.
- Carbon in CO: 1 lone pair (2 electrons) + half of 6 bonding (3) = 5. A free carbon owns 4. It has gained one, so formal charge −1.
- Oxygen in CO: 1 lone pair (2) + half of 6 bonding (3) = 5. A free oxygen owns 6. It has lost one, so formal charge +1.
The charges add to zero, as they must for a neutral molecule. Oxygen ends up +1 because it is the atom that donated the dative pair. And notice the slightly surprising result: the negative end sits on carbon, not on oxygen. Hold onto that, because it explains the next part.
Why it matters
- Bond strength. A triple bond is short and very strong. The C≡O bond is, in fact, the strongest bond of any diatomic molecule, stronger even than N₂. That is why CO is so stable and unreactive in ordinary conditions.
- The toxicity link (the bit students remember). Because the lone pair on carbon carries that −1 formal charge, carbon is the end that donates to metals. In your blood, CO uses that carbon lone pair to form a dative bond to the iron(II) at the centre of haemoglobin, and it binds more than 200 times as strongly as oxygen does. Once CO is on, oxygen cannot get on, and that is how carbon monoxide poisons you. The same dative-bonding trick that builds the molecule is the trick that makes it deadly.
Related species to practise
The dative bond is a recurring exam theme. Once CO clicks, try these:
- NO₃⁻ (nitrate), where the bonding is shared out across the oxygens.
- N₂O₄ and the nitrogen oxides, good for spotting dative bonds and odd-electron structures.
- NH₄⁺ and H₃O⁺, the classic “extra bond from a lone pair” ions.
If you can explain where every electron comes from in each of these, you have understood dative bonding properly.
Frequently asked questions
Why does carbon monoxide have a triple bond? Carbon brings 4 outer electrons and oxygen 6, giving 10 in total. To complete both octets, oxygen donates a lone pair as a dative bond, adding a third bond on top of two ordinary covalent bonds.
What is the dative bond in CO? It is the third bond, where both shared electrons come from oxygen rather than one from each atom. It lets carbon reach an octet and produces the −1 (carbon) and +1 (oxygen) formal charges.
Why is carbon monoxide toxic? The carbon end carries a −1 formal charge and a lone pair, so CO bonds datively to the iron(II) in haemoglobin more than 200 times as strongly as oxygen, blocking oxygen transport.
See it built, step by step
The free dot-and-cross simulation has a guided CO walkthrough that reveals the diagram one stage at a time, so you can watch the dative bond from oxygen complete both octets.
Book a trial lesson and we will make dot-and-cross second nature.