IB Chemistry HLTopic 6 — Electron Pair SharingPaper 1 & 2Materials~10 min read
Coordination Bonds
A transition metal ion in solution is never alone. It sits at the centre of a small crowd of molecules and ions, all donating lone pairs into it. Get the counting right and this topic becomes bookkeeping rather than chemistry.
📚 What you need to know
A complex is a central metal ion surrounded by ligands.
A ligand donates a lone pair to the metal, forming a coordinate bond.
Every ligand is therefore a Lewis base and a nucleophile; the metal is the Lewis acid.
Monodentate = one bond. Bidentate = two. Multidentate = more than two.
Coordination number = number of coordinate bonds, not the number of ligands.
Charge on the complex = metal charge + sum of ligand charges. Neutral ligands change nothing.
What a complex looks like
Drop copper(II) sulfate into water and you do not get a bare Cu2+ floating about. Six water molecules immediately arrange themselves around it, each pointing an oxygen lone pair inwards.
The square brackets in the formula are not decoration. They mark off everything that is bonded to the metal, and the charge written outside them belongs to the whole package.
Water is neutral, so all six ligands together add nothing to the charge. The 2+ outside the bracket is still just the copper.
The arrows all point inwards. That is not a stylistic choice — it records that the electrons started on the ligand and ended up shared with the metal. Draw them outwards and you have said the opposite of what you meant.
How many bites can a ligand take?
Some ligands only have one donor atom, so they can only grip the metal in one place. Others carry two or more donor atoms and can wrap around it, gripping in several places at once.
The word for this is denticity, from the Latin for tooth. Monodentate ligands bite once; bidentate ligands bite twice.
The orange chain on the right is the carbon backbone joining the two nitrogen donors. It is the reason one molecule can reach two positions.
The ligands you should recognise
Ligand
Formula
Denticity
Donor atom
Charge
Water
H2O
Monodentate
Oxygen
0
Ammonia
NH3
Monodentate
Nitrogen
0
Chloride
Cl–
Monodentate
Chlorine
1–
Cyanide
CN–
Monodentate
Carbon
1–
Hydroxide
OH–
Monodentate
Oxygen
1–
1,2-diaminoethane (“en”)
H2NCH2CH2NH2
Bidentate
Two nitrogens
0
Ethanedioate (“ox”)
C2O42–
Bidentate
Two oxygens
2–
EDTA
EDTA4–
Hexadentate
Two N, four O
4–
EDTA is the extreme case. One EDTA4– ion has six donor atoms, so a single molecule wraps right round a metal ion and fills all six positions by itself. Its full name is ethylenediaminetetraacetic acid, which is why nobody writes it out.
Coordination number and shape
Coordination number is simply how many coordinate bonds reach the metal. Count arrows, not molecules. Once you have the number, the shape usually follows.
These are flat drawings of three-dimensional shapes. The tetrahedral one really has all four ligands pointing away from each other in space.
Working out the charge on a complex
This is pure arithmetic, and it is a guaranteed mark if you are careful.
Charge on the complex
charge = charge on metal ion + (sum of ligand charges)
Metal ion
Ligand
Coordination number
Working
Formula
Cu2+
Cl–
4
(2+) + 4(1–) = 2–
[CuCl4]2–
Fe2+
H2O
6
(2+) + 6(0) = 2+
[Fe(H2O)6]2+
Fe3+
H2O
6
(3+) + 6(0) = 3+
[Fe(H2O)6]3+
Ag+
NH3
2
(1+) + 2(0) = 1+
[Ag(NH3)2]+
🧩 Reading a complex ion formula
Find the square brackets. Everything inside is bonded to the metal.
Anything outside the brackets is a spectator ion — it is not a ligand.
Count the donor atoms inside to get the coordination number.
Add up the ligand charges. Neutral ligands contribute zero.
Subtract from the total charge shown outside the bracket to find the metal’s oxidation state.
Check the shape against the coordination number.
Worked examples
WORKED EXAMPLE
Three chromium(III) compounds are shown: [Cr(H2O)6]Cl3, [CrCl(H2O)5]Cl2, [CrCl2(H2O)4]Cl. Give the charge on each complex ion.
Use the chlorides outside the bracket
Each Cl– outside must be balanced by the charge inside.
First compound
Three Cl– outside, so the complex must be 3+. Check: (3+) + 6(0) = 3+.
Second compound
Two Cl– outside, so 2+. Check: (3+) + 1(1–) + 5(0) = 2+.
Third compound
One Cl– outside, so 1+. Check: (3+) + 2(1–) + 4(0) = 1+.
3+, 2+, 1+the chromium is 3+ in all three — only the chloride ligands inside change things
WORKED EXAMPLE
[Co(C2O4)3]3– contains three ethanedioate ligands. State the coordination number and work out the oxidation state of cobalt.
Check the denticity
Ethanedioate is bidentate, so each one forms two bonds.
Coordination number3 ligands × 2 bonds = 6Now the charges
Three ligands at 2– each give 6– in total. The whole ion is 3–.
x + (6–) = 3– → x = 3+Coordination number 6, octahedral, cobalt(III)three ligands but six bonds — this is exactly the trap the question is testing
💡 Exam tip
Say coordinate bond (or dative covalent). “Ionic attraction” scores nothing here.
When defining a ligand, include that it donates a lone pair. That is the definition, not “sticks to the metal”.
Coordination number = number of bonds. For bidentate ligands, double before you answer.
Neutral ligands contribute zero to the charge. Do not accidentally add anything for water or ammonia.
Ions written outside the square brackets are never ligands.
Link it back: ligand = Lewis base = nucleophile. Examiners love that connection.
⚠️ Common mix-up
Counting ligands instead of bonds. Three bidentate ligands give a coordination number of six, not three.
Including counter-ions as ligands. In [Cr(H2O)6]Cl3 the three chlorides are outside and bonded to nothing.
Forgetting ligand charges. Cl– and CN– do change the total; H2O and NH3 do not.
Drawing coordinate arrows outwards. They point from ligand to metal.
Assuming 4 always means tetrahedral. Square planar is also a coordination number of 4.
Thinking the metal donates the electrons. The metal is the acceptor. It has the empty orbitals.
Up next: Nucleophilic Substitution in Halogenoalkanes — back to organic, and the two rival routes that give the same product by very different roads.
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