The shape of the lesson
By the end of the lesson, learners can
- Ligands and denticity: Each ligand uses a lone pair on a donor atom.
- Shape, charge and oxidation state: Use the complex shapes view in the simulator to revise octahedral, tetrahedral, square planar and linear arrangements, with the coordination number and overall charge of each complex.
- Ligand: lone pair donor, forms a dative bond.
- Denticity: mono (H2O), bi (en, C2O42−), hexa (EDTA4−).
- Shape: 6 octahedral; 4 tetrahedral or square planar; 2 linear.
- Charge: metal oxidation state + ligand charges.
One picture every idea returns to
A complex is a central metal atom or ion surrounded by ligands. A ligand is a species with at least one lone pair that forms a dative covalent (coordinate) bond to the metal. Monodentate ligands (H2O, NH3, Cl−, CN−, OH−) form one bond; bidentate ligands (1,2-diaminoethane, en, and ethanedioate, C2O42−) form two; polydentate EDTA4− forms six. The coordination number is the number of dative bonds to the metal. Coordination number 6 gives an octahedral complex, 4 gives tetrahedral (large ligands such as Cl−) or square planar (Pt2+, as in cisplatin), and 2 gives linear, as in [Ag(NH3)2]+. The overall charge is the metal's oxidation state plus the charges of the ligands.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Lone pairs around a metal ion.. A transition metal ion surrounded by molecules or ions that each donate a lone pair is a complex. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Ligands and denticity | Each ligand uses a lone pair on a donor atom. Project tab 2 of the lab (Transition elements lab, complex shapes) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Shape, charge and oxidation state | Use the complex shapes view in the simulator to revise octahedral, tetrahedral, square planar and linear arrangements, with the coordination number and overall charge of each complex. Project tab 2 of the lab (Transition elements lab, complex shapes) and let a student drive it while the class predicts. | Connect-Extend-Challenge |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Shape, charge and oxidation state, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The coordination number counts dative bonds, not ligands: [Cu(en) 2 (H 2 O) 2 ] 2+ is 6-coordinate. | Neutral ligands (H 2 O, NH 3, en) add nothing to the charge, so [Co(NH 3) 6 ] 3+ contains Co(+3). Four ligands can mean tetrahedral or square planar; Pt(II) complexes are square planar. |
Complex ions and ligands: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Define the terms (a) ligand, (b) complex, (c) coordination number.
For [Co(en)2Cl2]+, give (a) the oxidation state of cobalt, (b) the coordination number, (c) the shape.
Explain why 1,2-diaminoethane is described as a bidentate ligand.
Draw and name the shape of (a) [Cu(H2O)6]2+, (b) [CuCl4]2−, (c) [Ag(NH3)2]+, stating bond angles.
EDTA4− forms a 1 : 1 complex with Fe3+. Give the formula and charge of the complex and its coordination number.
Suggest why Cu2+ forms a six-coordinate complex with water but a four-coordinate complex with chloride ions.
Answer key and marking notes
- (a) A molecule or ion with a lone pair that forms a dative covalent bond to a central metal atom or ion.
- (b) A central metal atom or ion surrounded by ligands bonded to it by dative bonds.
- (c) The number of dative bonds formed between the ligands and the central metal atom or ion.
- (a) en is neutral and each Cl− is −1, so x − 2 = +1, x = +3.
- (b) 2 × 2 (en) + 2 × 1 (Cl) = 6.
- (c) octahedral.
- Each of its two nitrogen atoms has a lone pair.
- Both lone pairs form dative bonds to the same metal ion, so one ligand forms two bonds.
- (a) octahedral, 90°, six O donor atoms around Cu.
- (b) tetrahedral, 109.5°.
- (c) linear, 180°.
- [Fe(EDTA)]−: +3 + (−4) = −1.
- EDTA4− is hexadentate, so the coordination number is 6 (octahedral).
- Chloride ions are much larger than water molecules.
- Only four Cl− ions fit around the Cu2+ ion (and they repel each other), giving a tetrahedral complex.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What is a ligand?
- Aa molecule or ion with a lone pair of electrons that forms a dative (coordinate) bond to a central metal atom or ion
- Ba positive ion that accepts a lone pair from a metal atom
- Ca molecule that forms an ionic bond with a transition metal ion
- Dany molecule that is dissolved in water with a transition metal ion
What is the coordination number of the metal in [Cu(en)2(H2O)2]2+? (en = 1,2-diaminoethane, H2NCH2CH2NH2)
- A6
- B4
- C2
- D8
What is the shape of the [Ag(NH3)2]+ ion, the active complex in Tollens' reagent?
- Alinear, with an N-Ag-N bond angle of 180°
- Bbent, with a bond angle of about 104.5°, like water
- Ctetrahedral, with bond angles of 109.5°
- Dsquare planar, with bond angles of 90°
What is the shape of the [CuCl4]2− ion?
- Atetrahedral, bond angle 109.5°
- Bsquare planar, bond angle 90°
- Coctahedral, bond angle 90°
- Dtrigonal planar, bond angle 120°
An iron(III) ion bonds to six cyanide ions, CN−. What is the formula of the complex ion?
- A[Fe(CN)6]3−
- B[Fe(CN)6]3+
- C[Fe(CN)6]6−
- D[Fe(CN)6]9−
What is the oxidation state of cobalt in the complex ion [Co(EDTA)]−?
- A+3
- B−1
- C+5
- D−5
Ethanedioate, C2O42−, is a bidentate ligand. An octahedral complex forms between Fe3+ and ethanedioate only. How many ethanedioate ions does it contain, and what is the overall charge?
- A3 ions; overall charge 3−
- B6 ions; overall charge 9−
- C3 ions; overall charge 3+
- D2 ions; overall charge 1−
Which complex has coordination number 4 AND bond angles of 90° between adjacent ligands?
- A[Pt(NH3)2Cl2]
- B[CuCl4]2−
- C[Ag(NH3)2]+
- D[Co(NH3)6]3+
Quiz key
A ligand donates a lone pair to the metal, forming a dative covalent bond; the metal ion accepts the pair into empty orbitals. Common ligands are H2O, NH3, Cl−, CN− and OH−. A positive ion accepting electrons from the metal reverses the roles: the metal is the electron-pair acceptor. The metal-ligand bond is dative covalent, not ionic. A molecule merely dissolved alongside the ion is not a ligand unless it bonds to the metal through a lone pair.
Each en is bidentate: both of its N atoms donate a lone pair, giving 2 × 2 = 4 bonds, plus 2 bonds from the water molecules, so the coordination number is 6 (octahedral). 4 counts the ligands (2 en + 2 water) rather than the bonds. 2 counts only the en ligands. 8 counts each en as forming four bonds, but each en has only two donor N atoms.
[Ag(NH3)2]+ has coordination number 2 and is linear, angle 180°. The bent shape of water comes from two lone pairs on oxygen; the silver ion's d electrons do not bend this complex. Tetrahedral and square planar shapes both need coordination number 4, but silver here has only two ligands.
[CuCl4]2− has coordination number 4 and is tetrahedral (109.5°); the large Cl− ions cannot fit six around the Cu2+ ion. Square planar is the shape of [Pt(NH3)2Cl2] and is the classic confusion for four-coordinate complexes; in the 9701 syllabus [CuCl4]2− is the tetrahedral example. Octahedral needs six ligands, and trigonal planar needs three.
Overall charge = (+3) + 6 × (−1) = −3, so the ion is [Fe(CN)6]3− (hexacyanoferrate(III)). 3+ ignores the charge on the ligands, treating cyanide as neutral like H2O. 6− adds only the ligand charges and forgets the metal. 9− subtracts the metal charge instead of adding it.
Let the oxidation state be x. x + (−4) = −1, so x = +3. −1 is just the overall charge of the complex. +5 adds 4 and 1 instead of solving the equation (a sign error). −5 comes from x = −1 − 4, another sign error; a metal in a complex ion has a positive oxidation state here.
Six bonds / 2 per ligand = 3 ethanedioate ions, so the ion is [Fe(C2O4)3]3−: +3 + 3 × (−2) = −3. Six ions treats ethanedioate as monodentate. 3+ ignores the 2− charge on each ligand. Two ions would give only four bonds, so it would not be octahedral.
[Pt(NH3)2Cl2] is square planar: four ligands in one plane at 90° to their neighbours. [CuCl4]2− is also four-coordinate but tetrahedral (109.5°). [Ag(NH3)2]+ is linear with coordination number 2. [Co(NH3)6]3+ has 90° angles but is octahedral, coordination number 6.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.