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Spec Mapping — OCR H432 Module 5.3.1 — Transition elements, covering stereoisomerism of complex ions in the context of cis-trans (geometric) isomerism in square-planar [Ma2b2] and octahedral [Ma4b2] complexes, and optical (chiral) isomerism in octahedral complexes with three bidentate ligands or with two bidentate plus two monodentate ligands in a cis arrangement (refer to the official OCR H432 specification document for exact wording).
Stereoisomerism in transition-metal complexes is the synoptic bridge that links Year-12 organic cis-trans and optical isomerism to the three-dimensional geometry of [Man] coordination spheres. The OCR Module 5.3.1 specification expects you to recognise two distinct phenomena: cis-trans (geometric) isomerism, which depends on the adjacent vs opposite placement of identical ligands around a fixed coordination polyhedron, and optical isomerism, which depends on the chirality (non-superimposable mirror image) of the whole coordination sphere. These two phenomena occur in different geometries and for different ligand sets — the most common A-Level mistake is to confuse the two or to draw "cis-trans" for a tetrahedral complex, where it is geometrically impossible. This lesson sets out the rules, walks through the canonical examples ([Pt(NH3)2Cl2] cisplatin/transplatin, [Co(NH3)4Cl2]+, [Co(en)3]3+, [Cr(C2O4)3]3−), explains the underlying symmetry reasoning, and rehearses the AO2/AO3 routes by which OCR examines these structures on Paper 1 and Paper 3.
Key Definition — Stereoisomers are isomers with the same structural formula (same atoms, same connectivity) but a different spatial arrangement of those atoms. In transition-metal chemistry two sub-types arise: cis-trans (geometric) isomerism, where two identical ligands can be adjacent (cis, 90°) or opposite (trans, 180°) around a fixed polyhedron; and optical isomerism (chirality), where the complex has a non-superimposable mirror image (the two mirror-image forms are enantiomers and rotate plane-polarised light in opposite directions).
By the end of this lesson you should be able to:
Stereoisomers are molecules with the same structural formula (same atoms, same bonds) but a different arrangement in space. You met stereoisomerism for organic compounds in Year 12 (E/Z isomers of alkenes, optical isomers of amino acids). In transition metal chemistry the same two types apply:
A square planar complex with the formula [Ma2b2] (two ligands a and two ligands b) can exist as cis (the two a's adjacent, 90 degrees apart) or trans (the two a's opposite, 180 degrees apart).
The classic example is cisplatin / transplatin, [Pt(NH3)2Cl2]:
Square planar [Pt(NH3)2Cl2] is the defining A-Level example of cis-trans isomerism in a complex. You must be able to draw both forms clearly with wedge bonds or equivalent.
An octahedral complex with the formula [Ma4b2] (four a and two b) can also show cis-trans isomerism:
Example: [Co(NH3)4Cl2]+, which can be drawn as:
The two forms have different physical and chemical properties - they absorb light differently, have different solubilities, and in some cases different reactivities (similar to cisplatin).
An object is chiral if it has a non-superimposable mirror image. Your two hands are chiral - you cannot place your left hand on top of your right so that all fingers align. A chiral complex is one whose mirror image cannot be superimposed on the original by any rotation.
Chiral molecules exist as two enantiomers. They are identical in every respect except:
At A-Level you need to know that octahedral complexes with three bidentate ligands ([M(en)3]^n+, [M(C2O4)3]^n-) are chiral.
The three bidentate ligands wrap around the central metal ion in a propeller-like arrangement. Looking down one of the three-fold axes, the bidentate ligands spiral either clockwise or anticlockwise. The two arrangements (delta and lambda, or right-handed and left-handed) are non-superimposable mirror images.
graph LR
A["Left handed<br/>lambda<br/>anticlockwise propeller"] -->|mirror| B["Right handed<br/>delta<br/>clockwise propeller"]
A -.-> C{"Non-superimposable<br/>enantiomers"}
B -.-> C
Examples of chiral octahedral complexes OCR expects you to recognise:
Structure: H2N-CH2-CH2-NH2. Both nitrogens have a lone pair available for donation to the metal. When both coordinate to the same metal, they form a five-membered chelate ring with the M (one M - one N - one C - one C - one N).
A complex with en is:
Structure: -O-CO-CO-O-, i.e. the dianion of ethanedioic (oxalic) acid. Both oxygens of the two carboxylate groups donate lone pairs to the metal. This forms a five-membered chelate ring (M - O - C - C - O).
Ethanedioate carries a -2 charge, so it significantly changes the overall charge of the complex. For example:
When a polydentate ligand replaces monodentate ligands, the reaction is entropically favoured because more free particles are produced. For example:
[Ni(H2O)6]2+ + 3 en -> [Ni(en)3]2+ + 6 H2O
On the left: 4 particles. On the right: 7 particles. Delta S is positive.
This extra entropic driving force is called the chelate effect and explains why chelating (bidentate or polydentate) ligands form particularly stable complexes - they are more thermodynamically favoured than equivalent monodentate substitutions. The chelate effect is the basis of EDTA's effectiveness as a metal scavenger and of biologically important multi-dentate ligands.
EDTA4- (ethylenediaminetetraacetate ion) has a complex structure with:
Total: 6 donor atoms, so EDTA4- is a hexadentate ligand. It occupies all six coordination sites of an octahedral metal ion, wrapping around the metal in a cage-like structure.
Charge: EDTA has four negative charges (four carboxylates), so:
The chelate effect is maximised: replacing 6 water molecules with 1 EDTA releases 6 waters, giving a massive entropy gain (7 particles on each side of the equation but the hexadentate ligand has huge rotational constraint when free - overall delta S is very positive).
| Geometry | Formula | Isomerism |
|---|---|---|
| Square planar | [Ma2b2] | cis-trans |
| Square planar | [Mabcd] | more complex (OCR does not require) |
| Octahedral | [Ma4b2] | cis-trans |
| Octahedral | [Ma2b2c2] | cis-trans (multiple forms) |
| Octahedral | [M(en)3] or [M(bidentate)3] | optical (chiral) |
| Octahedral | [Ma4(en)] | no stereoisomers (en always cis) |
Question: Draw the possible stereoisomers of [Pt(NH3)2Cl2], [Co(NH3)4Cl2]+, and [Ni(en)3]2+.
Answer:
A common Year-13 misconception is to draw cis/trans isomers of a tetrahedral [Ma2b2] complex (e.g. [Zn(NH3)2Cl2]). This is geometrically impossible. In a tetrahedron, all four corners are equivalent by symmetry: any pair of corners is related by a 109.5° angle to every other pair, so there is no operational distinction between "adjacent" and "opposite". Swap the labels on any two corners and you get a molecule indistinguishable from the original by rotation. Therefore tetrahedral [Ma2b2] has only one structure and no cis-trans isomers.
Conversely, tetrahedral complexes can show optical isomerism — but only when all four ligands are different ([Mabcd], the tetrahedral chiral-centre case analogous to a chiral carbon CR1R2R3R4). OCR does not normally examine this case in transition-metal chemistry, but it is the same chirality logic as the four-substituent rule from Year-12 organic.
Square-planar complexes can show cis-trans isomerism for [Ma2b2] (cisplatin / transplatin, lesson 6) because the four corners are coplanar and an adjacent (cis, 90°) arrangement is geometrically distinct from an opposite (trans, 180°) arrangement. However, square-planar complexes cannot show optical isomerism, because the plane of the molecule is itself a mirror plane — every square-planar complex is superimposable on its mirror image by reflection through that plane. This is why cisplatin shows cis-trans isomerism but no chirality.
| Geometry | Cis-trans? | Optical? | Worked example |
|---|---|---|---|
| Square planar [Ma2b2] | Yes (cis and trans) | No (mol-plane is mirror) | [Pt(NH3)2Cl2] |
| Square planar [Mabcd] | No (only one arrangement geometrically distinct) | No | rare |
| Tetrahedral [Ma2b2] | No (all corners equivalent) | No | [Zn(NH3)2Cl2] |
| Tetrahedral [Mabcd] | No | Yes (chiral, four different ligands) | not in OCR core |
| Octahedral [Ma4b2] | Yes | No | [Co(NH3)4Cl2]+ |
| Octahedral [Ma2b2c2] | Yes (multiple cis/trans patterns) | Can be (cis-cis-cis case) | [Cr(H2O)2(NH3)2Cl2] |
| Octahedral [M(bidentate)3] | No (always cis by chelate constraint) | Yes (Δ and Λ enantiomers) | [Co(en)3]3+ |
| Octahedral [Ma2(bidentate)2] cis | Yes (cis/trans of monodentates) | Yes (cis form only is chiral) | cis-[Co(en)2Cl2]+ |
Memorise the diagonal: tetrahedral → only [Mabcd] is chiral; square planar → only [Ma2b2] shows cis-trans; octahedral → both phenomena possible, in the geometries listed.
The complex [Co(en)2Cl2]+ is one of OCR's favourite synoptic targets because it combines both stereoisomerism types in one structure.
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