Werner’s theory of coordination compounds

What is Werner’s theory of coordination compounds?

In plain words: in CoCl₃·6NH₃ the six ammonia molecules are not loosely tied on; they sit tightly around the cobalt ion as one unit, [Co(NH₃)₆]³⁺, and the three chloride ions float free outside it. Count how much free chloride a compound gives, and you learn how the rest is attached.

Chlorophyll, haemoglobin and vitamin B₁₂ are coordination compounds of magnesium, iron and cobalt (NCERT page 118). The transition metals, most of the d-block elements, form a large number of them (NCERT page 118).

Primary and secondary valences

• The experiment. Cobalt(III) chloride forms a series of compounds with ammonia. Excess silver nitrate, in the cold, precipitates all three chlorides from one of them but only part of the chloride from the others, as AgCl (NCERT pages 118–119):

• Werner’s explanation. Six groups, chloride ions or ammonia molecules or both, stay bonded to the cobalt ion in every case, and the bracketed unit remains substantially intact during the cold, prompt test. Conductivity in solution agrees (NCERT page 119, Table 5.1):

Type: the electrolyte that the solution conductivity corresponds to, as NCERT’s Table 5.1 puts it.

• Reading the conductivity. A 1 : 3 electrolyte gives four ions per formula unit, a 1 : 2 three and a 1 : 1 two, so this series has the expected broad conductivity order at comparable dilution. Conductivity also depends on ionic charges and mobilities; ion count alone is not a universal quantitative rule. One step further, CoCl₃·3NH₃ is [CoCl₃(NH₃)₃] (NCERT page 121): no chloride is outside the bracket, so it gives no AgCl and is a non-electrolyte (reasoned).

• Primary valence is the normally ionisable one, satisfied by negative ions: 3 for CrCl₃, 2 for CoCl₂ and PdCl₂ (NCERT page 118). Secondary valence is the number of groups bound directly to the metal: six in all four cobalt compounds (NCERT page 119). In modern terms the secondary valence is the coordination number, as NCERT’s third postulate says (page 119). Beyond the NCERT text: the primary valence corresponds to the metal’s oxidation state (Wikipedia, “Alfred Werner”).

• One chloride can count twice. In [CoCl(NH₃)₅]Cl₂ the primary valence is still 3, for cobalt(III), but only two chlorides are ionisable: the chloride inside the bracket satisfies a primary and a secondary valence at once. That is why NCERT calls primary valences “normally” ionisable (page 119). The AgCl count gives the ionisable chlorides, not the primary valence (reasoned).

• Isomers. The green and violet compounds have the same empirical formula, CoCl₃·4NH₃, but different properties: they are isomers (NCERT page 119).

Postulates of Werner’s theory

Werner’s main postulates (NCERT page 119):

1. In coordination compounds metals show two types of linkage (valence), primary and secondary.
2. The primary valences are normally ionisable and are satisfied by negative ions.
3. The secondary valences are non-ionisable; they are satisfied by neutral molecules or negative ions. The secondary valence equals the coordination number and was treated as fixed for a metal in this historical postulate. Modern chemistry allows different coordination numbers for the same metal in different complexes.
4. The ions or groups bound by the secondary linkages have characteristic spatial arrangements for each coordination number.

• Modern words. These arrangements are now called coordination polyhedra; the bracketed species are coordination entities or complexes, and the ions outside the brackets are counter ions (NCERT page 119).
• Shapes. Werner held that octahedral, tetrahedral and square planar shapes are the most common: [Co(NH₃)₆]³⁺, [CoCl(NH₃)₅]²⁺ and [CoCl₂(NH₃)₄]⁺ are octahedral, [Ni(CO)₄] tetrahedral and [PtCl₄]²⁻ square planar (NCERT page 119).
• NCERT slip (page 119): NCERT says Werner propounded his theory “in 1898”, but its own box on page 120 says the theory was formulated from 1890 to 1893. Beyond the NCERT text: Werner first proposed these structures in 1893 (Wikipedia, “Alfred Werner”). So 1898 is the inconsistent textbook date, not the historical date of the proposal. If a question follows NCERT’s line, expect 1898; otherwise give 1893. He won the Nobel Prize in Chemistry in 1913 (NCERT page 120).

Double salts and complexes

• Both form when two or more stable compounds combine in a fixed ratio (NCERT page 120).
• Double salts dissociate completely into simple ions in water: carnallite, KCl·MgCl₂·6H₂O; Mohr’s salt, FeSO₄·(NH₄)₂SO₄·6H₂O; potash alum, KAl(SO₄)₂·12H₂O (NCERT page 120).
• Stable complex ions remain substantially intact under ordinary conditions. Complex ions such as [Fe(CN)₆]⁴⁻ of K₄[Fe(CN)₆] do not dissociate into Fe²⁺ and CN⁻ ions (NCERT page 120); strictly, they release no appreciable free Fe²⁺ or CN⁻ under ordinary test conditions (reasoned). So a solution of Mohr’s salt gives the tests of Fe²⁺, NH₄⁺ and SO₄²⁻, while one of K₄[Fe(CN)₆] does not give the tests of Fe²⁺ (reasoned).

Solved examples

Common mistakes to avoid

Quick revision summary

• Werner: metals have a primary (normally ionisable) valence, satisfied by anions, and a secondary (non-ionisable) valence, satisfied by neutral molecules or anions and equal to the coordination number.
• CoCl_₃ with NH_₃: 3, 2, 1 and 1 mol AgCl from the 6-, 5- and two 4-ammonia compounds; six groups always on cobalt.
• Groups on the secondary valence have fixed shapes: octahedral, tetrahedral, square planar.
• Double salts release their constituent ions on dissolution; stable complex ions such as [Fe(CN)_₆]^⁴⁻ remain substantially intact in the ordinary tests.

Questions students ask

How do you find the primary and secondary valency of a metal in a complex?

To find the secondary valency of a metal in a complex, count the donor atoms bonded to it, since the secondary valence equals the coordination number (NCERT pages 119 and 121); beyond the NCERT text, the primary valency corresponds to the metal’s oxidation state, worked out from the charges (Wikipedia, “Alfred Werner”). In [Cu(NH₃)₄]SO₄, x + 4(0) − 2 = 0 gives a primary valency of 2, and the four NH₃ give a secondary valency of 4. Count each didentate ligand twice: [Ni(en)₃]Cl₂ has a primary valency of 2 but a secondary valency of 6 (reasoned).

What are the limitations of Werner’s theory?

Werner’s theory said which groups a metal holds and in what shape, but not why: NCERT lists three basic questions it could not answer, why only certain elements form coordination compounds, why the bonds in them are directional, and why the compounds have characteristic magnetic and optical properties, their colours among them (NCERT Section 5.5, page 128). Later theories (valence bond, crystal field, ligand field and molecular orbital theory) were put forward to explain the bonding, and NCERT treats the first two (page 128; see bonding in coordination compounds).

Mohr’s salt contains [Fe(H₂O)₆]²⁺ ions, so why is it not a complex compound?

Mohr’s salt counts as a double salt, not a complex compound, because in water it breaks up completely into the simple ions of iron(II) sulphate and ammonium sulphate (NCERT page 120). It is true, beyond the NCERT text, that each iron(II) ion in the crystal carries six water molecules arranged octahedrally, and these [Fe(H₂O)₆]²⁺ units are hydrogen-bonded to the NH₄⁺ and SO₄²⁻ ions (Wikipedia, “Ammonium iron(II) sulfate”). But iron(II) sulphate alone gives the same aqua ion in water, so it is just the form Fe²⁺ takes there, not a new entity made when the two salts combine (reasoned).

Which chapter has Werner’s theory, and is it in the JEE Main 2027 and NEET (UG) 2026 syllabus?

Werner’s theory is in Unit 5, Coordination Compounds, of NCERT Class 12 Chemistry Part I (Section 5.1, pages 118–120), and it is named in both syllabuses. Unit 12, Coordination Compounds, of JEE Main 2027 opens “Introduction to coordination compounds. Werner’s theory, ligands, coordination number, denticity, chelation”, and Unit 12, Co-ordination Compounds, of NEET (UG) 2026 lists the same topics. Both go on to the “IUPAC nomenclature of mononuclear co-ordination compounds”, isomerism, valence bond and crystal field bonding, and the importance of coordination compounds.

How did Werner work out that six-coordinate complexes are octahedral?

Werner worked out the octahedral shape by counting isomers: in NCERT’s words, “Using the property of isomerism, Werner predicted the geometrical shapes of a large number of coordination entities” (NCERT page 137). [CoCl₂(NH₃)₄]Cl was known in just two forms, green and violet (NCERT page 119), and two Cl among six positions can be placed in exactly two different ways on an octahedron but in three on a flat hexagon or on a trigonal prism (reasoned). Of these three shapes, only the octahedron fits two forms (reasoned).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Explain the bonding in coordination compounds in terms of Werner’s postulates. (NCERT Exercise 5.1.)

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   The metal shows a primary valence, normally ionisable and satisfied by negative ions, and a secondary valence, non-ionisable and satisfied by neutral molecules or negative ions; the secondary valence equals the coordination number, historically treated as fixed for a metal. In modern chemistry the same metal can have different coordination numbers in different complexes. The groups on the secondary valence form the coordination entity, written in square brackets, with a characteristic shape such as octahedral, tetrahedral or square planar (NCERT page 119).

2. Q2. FeSO₄ solution mixed with (NH₄)₂SO₄ in a 1 : 1 molar ratio gives the test of Fe²⁺, but CuSO₄ mixed with aqueous ammonia in a 1 : 4 molar ratio does not give the test of Cu²⁺. Explain. (NCERT Exercise 5.2.)

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   The first forms a double salt, Mohr’s salt, which dissociates completely in water, so Fe²⁺ ions are free. The second forms the complex ion [Cu(NH₃)₄]²⁺, which greatly lowers the concentration of uncomplexed aqueous Cu²⁺ under the stated conditions, so its usual simple-ion test is suppressed. Complex formation is an equilibrium; the free-ion concentration is not exactly zero (reasoned from NCERT page 120).

3. Q3. In NCERT Example 5.1, PtCl₂·2NH₃ gives no AgCl. What is its formula, and what is the secondary valence of platinum?

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   No free chloride, so both Cl are inside: [PtCl₂(NH₃)₂], a neutral complex with secondary valence 4 (NCERT Example 5.1, page 120).