IB Chemistry HLTopic 2 — Models of Bonding & StructurePaper 1 & 2Core idea~8 min read
Coordinate (Dative) Bonds
In an ordinary covalent bond both atoms chip in one electron each. In a coordinate bond, one atom pays for the whole thing. It sounds like a special case, but it is how the ammonium ion forms, how acids work in water, and how every transition metal complex is held together.
📘 What you need to know
In a coordinate (dative) bond, both electrons in the shared pair come from the same atom.
You need two things: one atom with a lone pair to donate, and one atom that is electron deficient — it has an unfilled outer shell or an empty orbital.
It is drawn as an arrow pointing from the donor to the acceptor.
Once it has formed it is identical to any other covalent bond — same strength, same length, no way to tell them apart.
Classic example: NH3 + H+ → NH4+, where nitrogen’s lone pair fills the hydrogen ion’s empty shell.
The whole species carries the charge, so it is written in brackets with the charge outside.
At HL, coordinate bonding is how ligands attach to a metal ion, as in [Al(H2O)6]3+.
The one thing that makes it different
Everything about a coordinate bond is the same as a normal covalent bond except its origin story. Two electrons end up shared between two nuclei, exactly as before. The only difference is that they both started on one atom.
How to spot one on a diagram
If a question asks you to show a coordinate bond, the arrow is the mark. A plain line is not wrong chemistry, but it does not answer the question that was asked.
The two ingredients you always need
A coordinate bond will not form unless both of these are present. Get into the habit of naming both when you explain one.
🎁
THE DONOR
An atom carrying a lone pair: nitrogen, oxygen and the halogens are the usual suspects.
📁
THE ACCEPTOR
An electron deficient atom or ion: H+, BF3, AlCl3, or a metal ion.
The ammonium ion, step by step
This is the example the IB uses most, so learn it properly rather than half remembering it.
Ammonia has a nitrogen with three bonds and one lone pair. A hydrogen ion, H+, is a bare proton — it lost its only electron, so its 1s shell is completely empty. Put them together and the deal is obvious: nitrogen has a spare pair, H+ has nowhere to put anything. Nitrogen donates both electrons and a fourth N–H bond appears.
Ammonia plus a hydrogen ion
Nitrogen ends up with four bonds instead of three and no lone pair left, which is why the ion is positive. The proton brought a charge but no electrons.
🤔 Why are all four N–H bonds identical?
Electrons do not carry a label saying who owned them. Once the fourth bond has formed, nitrogen is sharing four pairs and every one of them sits in an equivalent position around the atom. Measure the four N–H bonds and they come out the same length, the same strength, and at the same 109.5° angles. The dative arrow is a story about how the bond was made, not a description of what it is now.
Same idea, different molecule. Water does exactly the same thing. One of oxygen’s two lone pairs grabs an H+ to give the oxonium ion, H3O+. That is what is really happening every time you write “H+ in solution” in an acids question.
Where else coordinate bonds turn up
Species
Donor
Acceptor
Coordinate bonds
NH4+
Lone pair on N
H+, empty 1s
1
H3O+
Lone pair on O
H+, empty 1s
1
H3N→BF3
Lone pair on N
B, only 6 electrons
1
[Al(H2O)6]3+
Lone pair on each O
Al3+, empty orbitals
6
Al2Cl6
Lone pair on Cl
Al, only 6 electrons
2
🧠 Spotting them in an exam
Ask two questions of any suspicious bond. Does one atom have more bonds than you would expect? Nitrogen with four, oxygen with three. Is the species positively charged or is there an electron deficient atom involved? If the answer to both is yes, you are looking at a coordinate bond.
One thing to be careful about at HL: the aluminium hexaaqua ion is officially part of the transition metal complexes material. You are not expected to learn the details of metal complexes at SL — you just need to recognise that coordinate bonding is not limited to small molecules like NH4+.
Worked examples
WE 1
Explain how the bond between NH3 and H+ forms, and state how it compares with the other three N–H bonds [3]
Mark 1: identify the donor
Nitrogen in ammonia has a lone pair of electrons.
Mark 2: identify the acceptor and the transfer
H+ has an empty outer shell, so nitrogen donates both electrons to form a coordinate bond.
Mark 3: the comparison
Once formed it is identical to the other three — same length, same strength.
Lone pair donated to an empty shell; all four bonds identicalthe word “identical” is nearly always the third mark here
WE 2
Water reacts with H+ to give H3O+. How many lone pairs are left on the oxygen?
Step 1: start with water
O has 6 valence electrons: 2 bonding pairs to H, 2 lone pairs.
Step 2: use one lone pair for the coordinate bond
One lone pair becomes the third O–H bond.
2 − 1 = 1Step 3: check the electron count
3 bonding pairs + 1 lone pair = 4 pairs = 8 electrons around O. Octet intact.
1 lone pair remainsfour domains with one lone pair means H₃O⁺ is trigonal pyramidal, just like NH₃
WE 3
State the number of coordinate bonds in [Al(H2O)6]3+ and explain which atom donates
Step 1: read the formula
Six water molecules are attached to one Al3+ ion.
Step 2: identify donor and acceptor
Each water donates one lone pair from its oxygen. Al3+ has empty orbitals to accept them.
Step 3: count6 water molecules × 1 bond each = 66 coordinate bonds, all donated by oxygenthe 3+ charge is written outside the brackets because it belongs to the whole complex
💡 Exam tips
Name both partners. “A lone pair is donated” is half an answer. Say which atom donates and why the other one can accept.
Draw the arrow the right way round: from the atom with the lone pair towards the atom with the empty space.
Always add brackets and the charge when the product is an ion. Missing brackets is a very cheap way to lose a mark.
If you use dots and crosses, make both electrons of the coordinate bond match the donor atom’s symbol. That is the whole point of the diagram.
For shape questions, a coordinate bond counts as an ordinary bonding pair. NH4+ is tetrahedral at 109.5°.
Watch out for atoms with more bonds than usual — four bonds on nitrogen or three on oxygen is your signal.
⚠ Common mix-ups
Thinking a coordinate bond is weaker. It is exactly the same as any other single covalent bond once it has formed.
Saying electrons are transferred. They are donated into a shared pair. Transfer would make it ionic.
Putting the charge on the nitrogen atom in NH4+. The charge belongs to the ion as a whole.
Drawing the arrow backwards, from the acceptor to the donor. Follow the electrons.
Forgetting that H+ has no electrons at all. That is precisely why it needs someone else’s pair.
Assuming any lone pair will form one. There has to be an electron deficient partner as well — ammonia does not bond to methane.
Up next: Shapes of Molecules (VSEPR) — where every lone pair you have been carefully drawing finally earns its keep, and you start predicting bond angles instead of memorising them.
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