Question
State Werner’s theory of coordination compounds. Explain the concepts of primary valence and secondary valence with the example of CoCl·6NH.
(NCERT Class 12, Chapter 9)
Solution — Step by Step
Alfred Werner (1893) proposed that metals in coordination compounds show two types of valency:
-
Primary valence (ionisable valence): The number of charges on the metal ion, satisfied by negative ions. These are ionisable and can be detected by AgNO test (for Cl).
-
Secondary valence (coordination number): The number of ligands directly bonded to the metal ion. These are non-ionisable and are directed in space with a fixed geometry.
For CoCl·6NH, Werner proposed the structure:
- Primary valence = 3 (Co is in +3 oxidation state, satisfied by 3 Cl ions outside the coordination sphere)
- Secondary valence = 6 (six NH molecules directly bonded to Co inside the coordination sphere)
- Geometry: Octahedral (six ligands arranged around the central metal)
When dissolved in water:
- The compound dissociates as:
- All 3 Cl ions are ionisable → precipitated by AgNO as AgCl
- The solution gives a conductivity corresponding to 4 ions (1 cation + 3 anions)
Compare with CoCl·5NH = : only 2 Cl ionise; conductivity shows 3 ions.
| Compound | Formula | Ionisable Cl | Ions | Conductivity |
|---|---|---|---|---|
| CoCl·6NH | 3 | 4 | Highest | |
| CoCl·5NH | 2 | 3 | High | |
| CoCl·4NH | 1 | 2 | Low | |
| CoCl·3NH | 0 | 0 | None |
Why This Works
Werner’s genius was recognising that the same formula (CoCl·6NH) could be explained not as a simple mixture but as a compound with an inner coordination sphere (secondary valence) and outer ionic sphere (primary valence). This explained why different compounds with the same empirical formula showed different conductivities and different numbers of AgCl precipitates.
His theory earned the Nobel Prize in Chemistry (1913) — the first for inorganic chemistry.
Alternative Method — Modern Terminology
In modern terms: primary valence = oxidation state; secondary valence = coordination number. The coordination sphere is enclosed in square brackets, and counter-ions are written outside.
Quick method to find the number of ionisable Cl: dissolve the compound, add excess AgNO, and count the moles of AgCl precipitated per mole of compound. For CBSE boards, always write the conductivity evidence and AgNO test as proof of Werner’s theory.
Common Mistake
Students confuse primary and secondary valence. Primary valence is the oxidation state (ionisable, satisfied by counter-ions). Secondary valence is the coordination number (non-ionisable, satisfied by ligands). In , the secondary valence is 6 (not 3), and the primary valence is 3 (not 6).