Stereochemistry & Isomer Counting
Part 1 described which atoms are bonded to which. This part describes how those bonds are arranged in space — and why that arrangement can be the difference between a cancer drug and an inactive compound. It covers cis/trans and fac/mer geometrical isomerism, a systematic method for counting isomers that works every time, chirality at a metal centre and the Λ/Δ convention, the finer layer of chelate-ring conformation, how enantiomers are resolved and how that resolution is lost again, and the laboratory methods that tell one isomer from another. Two layers on every section: a slow, hand-held beginner path and a research-grade advanced/reference path.
The 6 sections in Part 2
- 1Geometrical isomerism — cis/trans and fac/mer Free below
- 2Counting isomers systematically
- 3Chirality at a metal centre — Λ and Δ
- 4Conformational isomerism of chelate rings
- 5Resolution and racemisation
- 6Telling the isomers apart in the laboratory
Geometrical isomerism — cis/trans and fac/mer
Free extractSection D.1 of Part 2, reproduced in full from the book — figures and all. No sign-in, no paywall on this section.
Which positions are next to each other and which are opposite — and the two naming systems that describe it.
Two ligands on a metal are either adjacent or opposite. That single distinction generates most of the geometrical isomerism in the subject.
trans: the two named ligands are opposite — 180° apart.
Square planar MA₂B₂
The simplest case, and the one that produced cisplatin. Four positions in a plane; two ligands of each kind. Either the two B ligands sit next to each other, or across from each other. Two isomers, no more.
Octahedral MA₄B₂
Six positions. Put four of one ligand and two of another. The two B ligands are again either adjacent or opposite. Two isomers. This is the case Werner used to establish the octahedron.
Octahedral MA₃B₃ — fac and mer
Three of each ligand needs different words, because “adjacent” and “opposite” no longer describe the situation. Again there are exactly two arrangements.
mer (meridional): the three lie on a meridian — a great circle passing through both poles. Two of them are mutually trans; the third is cis to both.
The quickest test on paper: are any two of the three identical ligands opposite each other? If no, it is fac. If yes, it is mer.
⚠ Common mistakes & exam traps
- cis/trans is meaningless for tetrahedral complexes. If a question offers cis and trans options for a tetrahedral species, both are wrong.
- MA₅B has no isomers. All six octahedral positions are equivalent until you place a second different ligand, so a single substitution can only give one compound. Likewise MA₆.
- “cis” must say cis to what. In a complex with three different ligand types, stating “the cis isomer” without naming the pair is ambiguous.
- fac/mer applies to MA₃B₃, not to MA₄B₂. Using the wrong pair of terms is a giveaway that the geometry has not been drawn out.
- Do not assume the trans isomer is the more stable. Both isomers of [Pt(NH₃)₂Cl₂] are isolable solids and neither converts spontaneously into the other at room temperature. Which one a synthesis delivers is decided kinetically, by the trans effect (Part 8), not by any general thermodynamic preference. And chelating ligands can make cis the only possibility — a bidentate ligand with a normal bite angle simply cannot span two trans positions.
Explain why [Co(en)₂Cl₂]⁺ exists as cis and trans isomers, but [Co(en)₃]³⁺ does not show geometrical isomerism. Medium
Bite-angle limits on which isomers can exist at all, and the trans-spanning ligands that break the usual rules.
The statement “a bidentate ligand cannot span two trans positions” is the single most useful shortcut in isomer counting, and it deserves to be stated precisely rather than as a slogan. A normal five- or six-membered chelate ring holds its of roughly 75–95° — close to 90° for diamines such as en, and as low as about 78° for rigid five-membered chelates such as 2,2′-bipyridine. That fits a cis pair (90°) and comes nowhere near a trans pair (180°). The limit is geometric, not electronic.
It is a strong rule, not an absolute one. Ligands with long, rigid backbones designed specifically for the purpose — so-called trans-spanning ligands, typically diphosphines with a large bite angle — can bridge two trans positions. They are laboratory constructions rather than syllabus examples, but their existence is the reason the rule should be quoted as “an ordinary chelate cannot span trans”.
Why does chelation force cis geometry in cisplatin analogues?
This has a real therapeutic consequence. Because a chelate can only occupy cis positions, building one into a cisplatin analogue locks the complex in the cis configuration. The two clinical successors do it at opposite ends of the molecule, which is worth getting right because the two are routinely conflated. Carboplatin keeps cisplatin’s two monodentate ammine ligands and replaces the two chlorides with a single bidentate cyclobutane-1,1-dicarboxylate — so its chelate is the leaving group, and it also slows the aquation step. Oxaliplatin does the opposite: it replaces the two ammines with the bidentate diamine (1R,2R)-cyclohexane-1,2-diamine and uses bidentate oxalate as the leaving group, so both ends are chelates.
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Read the rest of Part 2
The remaining 5 sections of this part — Counting isomers systematically, Chirality at a metal centre — Λ and Δ, Conformational isomerism of chelate rings, Resolution and racemisation — and all nine parts of Coordination Chemistry are part of ChemVidya Full Access, along with the other books, 55 Study Notes and 6,000+ practice questions.
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