Use d-electron configurations, ligand denticity and coordination geometry to explain transition-metal complexes and their isomers.
Why an incomplete d sublevel matters
The AQA characteristic-properties comparison covers titanium to copper. Transition-metal chemistry is associated with incomplete d sublevels in atoms or ions: many oxidation states, complexes, coloured species and catalytic activity. For the usual classification question, identify a stable ion with a partially filled d sublevel rather than simply saying “it is in the d block”. Sc³⁺ is d⁰ and Zn²⁺ is d¹⁰; these common ions lack the characteristic partly filled d sublevel. Cu²⁺ is d⁹, so copper qualifies even though the copper atom has 3d¹⁰.
For first-row transition-metal cations, remove 4s electrons before 3d electrons. The neutral Cr and Cu configurations are exceptions to the simple filling pattern: [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹. An oxidation number is a formal electron-accounting value; a complex containing M(III) need not have an overall 3+ charge.
| Atom | Neutral configuration | Selected ion and configuration |
|---|---|---|
| Ti | 3d² 4s² | Ti³⁺: 3d¹; Ti⁴⁺: 3d⁰ |
| V | 3d³ 4s² | V²⁺: 3d³; V³⁺: 3d² |
| Cr | 3d⁵ 4s¹ | Cr³⁺: 3d³ |
| Mn | 3d⁵ 4s² | Mn²⁺: 3d⁵ |
| Fe | 3d⁶ 4s² | Fe²⁺: 3d⁶; Fe³⁺: 3d⁵ |
| Co | 3d⁷ 4s² | Co²⁺: 3d⁷ |
| Ni | 3d⁸ 4s² | Ni²⁺: 3d⁸ |
| Cu | 3d¹⁰ 4s¹ | Cu⁺: 3d¹⁰; Cu²⁺: 3d⁹ |
A ligand donates an electron pair
A complex contains a central metal atom or ion surrounded by ligands attached through coordinate bonds. A ligand is an ion or molecule that donates a lone pair to the metal; both electrons in the coordinate bond originate from that ligand. The ligand acts as a Lewis base and the accepting metal centre as a Lewis acid.
Coordination number is the number of coordinate bonds to the central metal, not necessarily the number of ligand molecules. Six water ligands, three bidentate ligands or one hexadentate ligand can each give coordination number six. Water binds through O, ammonia through N, and chloride through Cl. Keep the donor atom next to the metal when drawing bonds.
Count donor atoms on each ligand
For [Cr(C₂O₄)₃]³⁻, three oxalate ligands contribute −6, so chromium is +3. There are three ligands but six donor bonds. For [Cu(EDTA)]²⁻, +2 + (−4) = −2 and one EDTA ligand supplies six donor sites. Denticity describes binding to one metal, not the total number of lone pairs a molecule possesses.
| Ligand | Charge | Donor sites used for one metal |
|---|---|---|
| H₂O | 0 | One oxygen: monodentate |
| NH₃ | 0 | One nitrogen: monodentate |
| Cl⁻ | −1 | One chlorine: monodentate |
| H₂NCH₂CH₂NH₂, abbreviated en | 0 | Two nitrogen atoms: bidentate |
| C₂O₄²⁻, ethanedioate/oxalate | −2 | One oxygen from each carboxylate group: bidentate |
| EDTA⁴⁻ | −4 | Two nitrogen and four oxygen atoms: hexadentate in the examples here |
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Bidentate and EDTA donor structures to add
Labels to include:
- en: H₂N–CH₂–CH₂–NH₂ with lone pair on each N
- Oxalate: ⁻O–C(=O)–C(=O)–O⁻; donor O atoms from different carboxylate groups
- EDTA⁴⁻: (⁻OOC–CH₂)₂N–CH₂–CH₂–N(CH₂–COO⁻)₂
- Two N and four O donor arrows directed to the metal
- Five-membered chelate rings for en and oxalate; complex brackets and charge
Draw each full ligand before folding it around a metal. Mark the exact donating atoms and show six donor bonds for EDTA, not one bond merely because it is one molecule.
Coordination number narrows the possibilities
Coordination number four does not prove tetrahedral geometry: the platinum example is square planar. Large chloride ligands often favour coordination number four rather than six. Geometry also depends on the metal and its electronic structure; ligand size is not a universal prediction rule.
| Example | Coordination number | Shape and characteristic angles |
|---|---|---|
| [Fe(H₂O)₆]²⁺ | 6 | Octahedral: 90° between adjacent bonds and 180° opposite |
| [Cu(NH₃)₄(H₂O)₂]²⁺ | 6 | Distorted octahedral; two water ligands remain |
| [CoCl₄]²⁻ | 4 | Tetrahedral: about 109.5° |
| [Pt(NH₃)₂Cl₂] | 4 | Square planar: 90° and 180° |
| [Ag(NH₃)₂]⁺ | 2 | Linear: 180° |
Distinguish geometrical and optical isomers
Square-planar [Pt(NH₃)₂Cl₂] has a cis arrangement with the two chlorides adjacent and a trans arrangement with them opposite. Cisplatin is the cis form; its geometry permits binding to DNA in a way that disrupts cell division. Its medical use is not a claim that it targets only cancer cells.
Octahedral [Co(NH₃)₄Cl₂]⁺ also has cis and trans forms, with the chloride positions 90° or 180° apart. Tetrahedral complexes with two pairs of identical monodentate ligands do not show this cis/trans pair: all ligand positions are equivalent by rotation.
[Cr(en)₃]³⁺ has two non-superimposable mirror-image arrangements of its three chelate rings. These are optical isomers. Each rotates plane-polarised light in opposite directions under identical conditions; an equal mixture has no net rotation. Do not call every drawing that looks mirrored an optical pair: try rotations to test superimposability.
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Complex geometries and isomer pairs to add
Labels to include:
- Octahedral: four bonds in a plane, one wedge and one dashed bond
- Square-planar cis/trans Pt(NH₃)₂Cl₂: Cl adjacent/opposite
- Octahedral cis/trans [Co(NH₃)₄Cl₂]⁺: Cl positions 90°/180°
- Tetrahedral [CoCl₄]²⁻ with four Cl donor atoms
- Linear [Ag(NH₃)₂]⁺ with N donors
- [Cr(en)₃]³⁺ mirror pair: join each pair of N donors with its own ligand loop
- Brackets and charges on all complex ions; neutral platinum complex has no ionic charge
Use consistent perspective and draw both isomers explicitly. The optical pair needs the three connected bidentate ligands, not six disconnected N labels. Preserve metal, ligand identities and overall charge in every drawing.
Distinguish a ligand from a counter-ion
In [Cr(H₂O)₅Cl]Cl₂·H₂O, one chloride is bound inside the complex and two are external counter-ions. The complex has charge 2+ and chromium is +3; the water after the dot is water of crystallisation. A rapid silver-nitrate test, under conditions where coordinated chloride is not released during the test, precipitates the two free chlorides. Long reaction times or ligand exchange can invalidate that simple count.
Exam connection: June 2023 Paper 1 Q01 required complex definitions, oxidation-state accounting and clear isomer drawings. Write Cl for a chloride ligand joined to the metal, not Cl₂; show the complete complex charge.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Give the configurations of Fe²⁺ and Fe³⁺, starting from Fe = [Ar] 3d⁶ 4s².Show answer
Remove both 4s electrons first: Fe²⁺ is [Ar] 3d⁶. Remove a further 3d electron for Fe³⁺: [Ar] 3d⁵.
Q2. Find chromium’s oxidation state, ligand count and coordination number in [Cr(C₂O₄)₃]³⁻.Show answer
x + 3(−2) = −3, so chromium is +3. There are three bidentate ligands and six coordinate bonds: coordination number six.
Q3. A complex has four coordinate bonds. Must its shape be tetrahedral?Show answer
No. [CoCl₄]²⁻ is tetrahedral but [Pt(NH₃)₂Cl₂] is square planar. Identify the metal/ligand system or use the geometry given.
Q4. Explain the difference between cis/trans [Co(NH₃)₄Cl₂]⁺ and the optical pair of [Cr(en)₃]³⁺.Show answer
Cis/trans differ in whether the chlorides are adjacent or opposite in an octahedron. The two [Cr(en)₃]³⁺ arrangements are non-superimposable mirror images produced by the connected chelate rings.
Q5. Under a rapid free-chloride test, how many moles of AgCl form per mole of [Cr(H₂O)₅Cl]Cl₂·H₂O?Show answer
Two moles, from the two external Cl⁻ counter-ions. The coordinated chloride is not counted if it remains bound under the stated test conditions.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- AQA 7405 inorganic chemistry specification — 3.2.5 coverage and required skills.
- Chemrevise: Transition metals — Coverage checklist; explanations, data exercises and quick checks on this page are original Finesse material.
- AQA June 2023 Paper 1 mark scheme — Q01 pp11–12: complex definitions, oxidation states and isomers; Q07.7–07.8 pp24–25: EDTA donor atoms and back-titration. Paired with report Q01 p3 and Q07 p5.
- AQA June 2023 Paper 1 examiner report — Read alongside the question-specific marking guidance; not a universal wording checklist.
- AQA June 2022 Paper 1 mark scheme — Q07.1–07.7 pp36–37: colour, photon energy, ligand substitution and EDTA charge; Q03.1–03.2 pp19–20: peroxide titration and end point.
- AQA June 2022 Paper 1 examiner report — Q07 p5 and Q03 p3, read with the corresponding mark scheme. Original practice questions are not official AQA questions.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.