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The coordination compounds chapter starts with Werner's theory, and many students rush past it to get to naming. That is a mistake, because the theory is a piece of detective work that tells you how to read the formulas. Alfred Werner, working in Zurich, explained a set of puzzling cobalt compounds by putting the metal at the centre and the other groups around it in space. This post covers his evidence, works the silver chloride calculation that NCERT uses, and shows how he counted isomers to fix the shape.
Who was Alfred Werner?
The Nobel Prize facts page records that Werner was born on 12 December 1866 in Mulhouse, then in Alsace, and died on 15 November 1919 in Zurich. His Nobel biography says he studied in Zurich, became an associate professor at the University of Zurich in 1893 and full professor in 1895, and published about 150 papers himself on these compounds over the next 20 years.
In 1893 he published his paper on the constitution of inorganic compounds, Beitrag zur Konstitution anorganischer Verbindungen, in the Zeitschrift für anorganische Chemie. Note that the NCERT chapter says "Werner in 1898, propounded his theory"; the founding paper is dated 1893, and the Nobel page gives that year too.
What puzzle was he solving?
Cobalt(III) chloride combines with ammonia to give several compounds with different colours. The NCERT Class 12 chapter Coordination Compounds gives the key data. Adding excess silver nitrate in the cold precipitates only some of the chloride as AgCl:
1 mol CoCl₃·6NH₃ (yellow) gave 3 mol AgCl.
1 mol CoCl₃·5NH₃ (purple) gave 2 mol AgCl.
1 mol CoCl₃·4NH₃ (green) gave 1 mol AgCl.
1 mol CoCl₃·4NH₃ (violet) gave 1 mol AgCl.
Werner's explanation was that six groups in all stay bonded to cobalt. Any chloride that is one of those six is held tightly and does not precipitate. The rest are free ions. So the formulas are [Co(NH₃)₆]Cl₃, [CoCl(NH₃)₅]Cl₂ and [CoCl₂(NH₃)₄]Cl. Conductivity agrees: NCERT lists them as 1:3, 1:2 and 1:1 electrolytes.
A worked example with AgCl
Sample numbers: a solution contains 0.0100 mol of a cobalt(III) chloride-ammonia complex. Excess AgNO₃ gives 2.87 g of AgCl. Which complex is it?
Molar mass of AgCl = 107.87 + 35.45 = 143.32 g/mol.
Moles of AgCl = 2.87 / 143.32 = 0.0200 mol.
Ratio = 0.0200 / 0.0100 = 2. Two chlorides per formula are free ions.
Cobalt(III) needs three chlorides in total, so one is inside the coordination sphere, and to make six groups around cobalt there must be five NH₃: [CoCl(NH₃)₅]Cl₂.
If the precipitate had been 4.30 g, that is 0.0300 mol and three free chlorides, so the compound is [Co(NH₃)₆]Cl₃. For 1.43 g, one free chloride: [CoCl₂(NH₃)₄]Cl.
The most common mistake is to assume every chloride in the formula precipitates. Only the ions outside the square brackets do. Another is to confuse the coordination number (6 here) with the oxidation state of cobalt (+3).
How did Werner prove the shape was an octahedron?
The green and violet compounds have the same formula, CoCl₃·4NH₃, but different properties. They are isomers. Werner used this to test possible shapes. In his Nobel lecture he compared three symmetrical arrangements for six groups: a flat hexagon, a prism and an octahedron. For a compound with four groups of one kind and two of another, the plane and the prism each predict three isomers, but "the octahedron arrangement only envisages the existence of two." Only two forms were ever found: one with the two chlorides on neighbouring corners (cis), one with them opposite (trans).
That is the same reasoning van 't Hoff used for carbon, which our post on van 't Hoff and the tetrahedral carbon explains.
Some chemists still doubted him, arguing that carbon in the organic ligands might be the cause of the isomerism. Werner answered that even the simplest dichlorotetrammine cobalt salts show it. The strongest evidence was optical activity. An octahedral complex with the right ligands should exist as two mirror images. Werner's group reported such optically active cobalt complexes in papers such as Zur Kenntnis des asymmetrischen Kobaltatoms (on the asymmetric cobalt atom, 1911). The Nobel biography records that more than 40 series of optically active complexes were eventually separated, so the octahedral arrangement "was established as firmly as that of the tetrahedral carbon atom of van 't Hoff and Le Bel."
Why does this matter for your exam?
The JEE (Main) 2026 syllabus lists "Werner's theory, ligands, coordination number, denticity, chelation, IUPAC nomenclature of mononuclear co-ordination compounds, isomerism" in Unit 12; check the current year's syllabus. Once you read formulas the way Werner did, naming and isomer questions get easier; our guide to naming coordination compounds takes it from there.
If ideas such as coordination number and denticity still blur together, the Fermi Concept Library covers every concept in chemistry, each with its own lesson.
What can a student take from Werner's way of working?
Start from the awkward data. Same formula, different colours and different AgCl results: that was the clue.
Make the theory predict a number. Two isomers, not three, decided the shape.
Answer the strongest objection directly. When critics blamed carbon, he showed isomerism in simpler compounds and then optical activity.
Tonight, write the formulas of the four NCERT cobalt compounds in square-bracket form and say, for each, how many moles of AgCl one mole gives and why.
