Foundations, Nomenclature & Structural Isomerism
The first part of the deepest treatment of coordination chemistry ChemVidya has attempted. It deliberately contains no bonding theory: Werner’s framework and the evidence that forced it, the vocabulary of the coordination sphere, ligand classification and the chelate effect, oxidation state and electron counting, coordination number and geometry, IUPAC nomenclature, and structural isomerism — every one of which is a prerequisite for the crystal field and ligand field theory that follows in Parts 3 and 4. Two layers on every section: a slow, hand-held beginner path and a research-grade advanced/reference path, so one book carries a reader from “what is the bracket for” to “why does bite angle change catalytic selectivity”.
The 14 sections in Part 1
- 1Werner’s two valences Free below
- 2The two experiments that decide a formulation
- 3The coordination sphere, and the vocabulary that goes with it
- 4Ligands — how they are classified and why it matters
- 5Chelation and the chelate effect
- 6Oxidation state and the d-electron count
- 7EAN and the 18-electron rule
- 8Coordination number and geometry
- 9Naming a coordination compound
- 10Writing the formula
- 11Ionisation and hydrate isomerism
- 12Coordination and coordination-position isomerism
- 13Linkage isomerism
- 14The nine parts at a glance
Werner’s two valences
Free extractSection A.1 of Part 1, reproduced in full from the book — figures and all. No sign-in, no paywall on this section.
The one idea that makes coordination compounds make sense — taught slowly, because everything later depends on it.
Before Werner, the accepted picture was the chain theory of Blomstrand and Jørgensen: the extra ammonia molecules were imagined to link together into –NH₃–NH₃– chains hanging off the metal, rather like the carbon chains that were proving so successful in organic chemistry. It was a reasonable guess. It was also wrong, and the way it failed is instructive.
Werner proposed instead that a metal ion exercises two independent valences:
Two rules follow, and Werner stated both:
2. The secondary valence is directed to fixed positions in space.
The second rule is the radical one. It asserts that a coordination compound has a definite three-dimensional shape — and therefore that it can show isomerism. That prediction is what eventually settled the argument, and we come to it in A.3.
The cobalt–ammine series. Werner’s key evidence was a family of compounds with the same two components in different ratios. Written the old way they look like a puzzle; written the Werner way they form an obvious series in which the secondary valence stays fixed at six while chloride steps from outside the sphere to inside it, one at a time.
| Old formula | Werner formulation | Ions in solution | Cl⁻ precipitated by AgNO₃ | Historic name & colour |
|---|---|---|---|---|
| CoCl₃·6NH₃ | [Co(NH₃)₆]Cl₃ | 4 (1:3) | 3 | luteo — yellow/orange |
| CoCl₃·5NH₃ | [Co(NH₃)₅Cl]Cl₂ | 3 (1:2) | 2 | purpureo — purple |
| CoCl₃·4NH₃ | [Co(NH₃)₄Cl₂]Cl | 2 (1:1) | 1 | praseo (trans) green; violeo (cis) violet |
| CoCl₃·3NH₃ | [Co(NH₃)₃Cl₃] | 0 | 0 | non-electrolyte |
Read the table down the right-hand columns and the argument writes itself. If the three chlorides in each compound were simply lattice chloride, every member of the series would release three chloride ions. They do not. The number falls 3, 2, 1, 0 in exact step with the number of ammonias replaced. Something is taking chloride out of circulation, one at a time, and that something is the coordination sphere.
⚠ Common mistakes & exam traps
- Primary valence is not “the first bonds formed”. It is simply the oxidation state. It has no geometry and no ordering in time.
- Secondary valence ≠ number of ligand molecules when chelates are present. A bidentate ligand occupies two secondary-valence positions with one molecule. [Co(en)₃]³⁺ has coordination number 6, not 3.
- Coordination number is a property of the metal centre, not of the compound. In K₃[Fe(CN)₆] the coordination number is 6 — the three potassium ions are irrelevant to it.
- Do not read the old dot formulae as hydrates or adducts. CoCl₃·6NH₃ is not “CoCl₃ with ammonia stuck on”; the dot notation simply records analysis, not structure.
Why the chain theory really lost, what Werner’s two valences became in modern bonding language, and where the primary/secondary distinction breaks down.
It is worth being precise about why Jørgensen’s chain theory failed, because the usual textbook account — “Werner was right, Jørgensen was wrong” — hides the actual logic. The chain theory was not naive: by placing chlorides at the ends of ammonia chains it too distinguished bound from ionic chloride, and it reproduced most of the conductivity data. It broke on two fronts. The first was the neutral triammine [Co(NH₃)₃Cl₃], the last row of the table above: a chain formulation demands at least one ionisable anion, whereas Werner predicted — and Werner and Miolati measured — a non-electrolyte. The second, and decisive, was isomer counting.
For a compound MA₄B₂, the number of distinguishable isomers depends on the geometry assumed, and the three candidate geometries for coordination number six give different answers:
| Geometry for CN 6 | Predicted isomers of [MA₄B₂] | Observed? |
|---|---|---|
| Planar hexagon | 3 (1,2- ; 1,3- ; 1,4-) | No — only 2 are ever found |
| Trigonal prism | 3 | No |
| Octahedron | 2 (cis, trans) | Yes |
A negative result of this kind is weak evidence on its own — a third isomer might simply be hard to make. Werner therefore closed the argument with a positive one: an octahedral cis-[M(AA)₂X₂] complex is chiral (it has no improper axis), whereas the planar hexagon is necessarily achiral — a planar molecule always has the molecular plane as a mirror — and the prismatic arrangements available to these chelates are achiral too. In 1911 he resolved cis-[Co(en)₂(NH₃)Cl]²⁺ into optical antipodes. That resolution is the direct experimental proof of the octahedron.
The hexol demonstration deserves its own mention because of what it was designed to rule out. A sceptic could argue that any optical activity observed in an ammine complex came from the organic ligand (ethylenediamine), not from the metal geometry. Werner therefore resolved a complex containing no carbon at all — the tris-chelate [Co{(μ-OH)₂Co(NH₃)₄}₃]⁶⁺, in which three Co(NH₃)₄ units are bound to a central cobalt through pairs of bridging hydroxides. Optical activity in a purely inorganic molecule can only come from the arrangement of ligands about the metal. The argument was then closed.
What the two valences became
Werner had no electronic theory — when he published in 1893 the electron had not yet been discovered, and the Bohr atom lay twenty years in the future. descriptions of behaviour, not mechanisms. Modern bonding theory reinterprets them cleanly:
| Werner’s term | Modern reading | Where it is developed |
|---|---|---|
| Primary valence | Formal oxidation state; the ionic charge requiring counter-ions | A.7 of this part |
| Secondary valence | Coordination number — the number of σ-donor sites accepted by the metal’s available acceptor orbitals | A.8, then Parts 3–4 |
| “Directed to fixed positions” | The geometry set by minimising ligand–ligand repulsion and maximising metal–ligand orbital overlap | Part 3 (CFT), Part 4 (LFT/MO) |
Where does the distinction break down? In three places worth knowing, because examiners like them:
- Neutral-metal complexes. In Ni(CO)₄ the oxidation state is zero, so the primary valence is zero, yet four ligands are firmly bound. Werner’s scheme has no trouble with this in principle, but it shows that primary valence carries no information about bond strength.
- Non-innocent ligands. For ligands such as dithiolenes, NO or o-quinones the oxidation state cannot be assigned unambiguously at all — the electron density is genuinely delocalised between metal and ligand, and “primary valence” loses its meaning. This is discussed in Part 4.
- Metal–metal bonded and cluster compounds. When metals bond to each other, a coordination number counted only over ligands understates the true connectivity. Part 9 returns to this.
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Further reading. Cotton & Wilkinson Advanced Inorganic Chemistry, Greenwood & Earnshaw Chemistry of the Elements, Housecroft & Sharpe Inorganic Chemistry, Miessler, Fischer & Tarr Inorganic Chemistry, Huheey, Keiter & Keiter Inorganic Chemistry, and the IUPAC Red Book (2005).
Read the rest of Part 1
The remaining 13 sections of this part — The two experiments that decide a formulation, The coordination sphere, and the vocabulary that goes with it, Ligands — how they are classified and why it matters, Chelation and the chelate… — 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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