Question
Werner proposed his coordination theory in 1893 to explain why compounds like CoCl₃·6NH₃ exist and behave the way they do. The classic question:
Explain Werner’s theory of coordination compounds, distinguishing between primary and secondary valency. Give an example with [Co(NH₃)₆]Cl₃.
Solution — Step by Step
Werner observed that transition metals show two different kinds of combining capacity. He called these primary valency (ionisable, also called oxidation state) and secondary valency (non-ionisable, also called coordination number).
Primary valency satisfies the charge of the metal ion — it is shown by ions that ionise in solution. Secondary valency satisfies the directional bonding around the metal — it is shown by ligands held inside the coordination sphere.
In [Co(NH₃)₆]Cl₃, cobalt has a primary valency of 3. This is satisfied by the three Cl⁻ ions sitting outside the square bracket. These chloride ions are free to ionise in solution.
When you do a AgNO₃ test on this compound, all three chlorides precipitate immediately. That’s your proof they are ionisable.
The six NH₃ molecules inside the bracket satisfy cobalt’s secondary valency of 6. These are directly bonded to Co and do NOT dissociate in solution. No matter how much AgNO₃ you add, these NH₃ ligands stay put.
Secondary valency is always fixed for a given metal — cobalt almost always shows a coordination number of 6. Primary valency varies with oxidation state.
Everything inside the square bracket [Co(NH₃)₆] is the coordination sphere (or inner sphere). The species outside — 3Cl⁻ — form the ionisation sphere (outer sphere).
Werner’s rule: primary valency is satisfied by anions that can be either inside or outside the coordination sphere. Secondary valency is satisfied only by species inside the coordination sphere.
Werner verified his theory by testing conductance and AgNO₃ precipitation across a series of cobalt-ammonia complexes:
| Compound | Formula | Cl⁻ ions free | Conductance |
|---|---|---|---|
CoCl₃·6NH₃ | [Co(NH₃)₆]Cl₃ | 3 | High (4 ions) |
CoCl₃·5NH₃ | [Co(NH₃)₅Cl]Cl₂ | 2 | Medium (3 ions) |
CoCl₃·4NH₃ | [Co(NH₃)₄Cl₂]Cl | 1 | Low (2 ions) |
CoCl₃·3NH₃ | [Co(NH₃)₃Cl₃] | 0 | None (non-electrolyte) |
This pattern is Werner’s strongest experimental evidence. The coordination number stays 6 throughout — some spots just get filled by Cl instead of NH₃.
Why This Works
The central insight is that transition metals have a specific geometry around them determined by their secondary valency. Co³⁺ always wants 6 things around it — whether those 6 things are all NH₃ or a mix of NH₃ and Cl⁻ doesn’t matter. The geometry is octahedral.
Primary valency behaving as charge is straightforward: Co³⁺ needs three negative charges to become neutral, satisfied by 3 Cl⁻. But where those Cl⁻ sit — inside or outside the bracket — changes the compound’s properties entirely. That’s why CoCl₃·6NH₃ and CoCl₃·3NH₃ look similar on paper but behave completely differently in solution.
Werner proposed this without any knowledge of atomic orbitals or bonding theory — he worked purely from conductance data and precipitation tests. It’s a genuinely elegant piece of reasoning.
Alternative Method — Using the Bracket Notation Directly
If you’re given the full IUPAC formula, just count:
- Write down what’s inside
[ ]— that’s your coordination sphere - Count the ligands — that’s the coordination number (secondary valency)
- Find the metal’s charge from what’s outside — that’s the oxidation state (primary valency)
For [Co(en)₂Cl₂]Cl — en (ethylenediamine) is a bidentate ligand, occupies 2 spots each. So: 2 en × 2 + 2 Cl = 6. Coordination number = 6. One Cl outside → Co³⁺. Primary valency = 3.
Bidentate ligands count twice toward coordination number. en, ox²⁻, and C₂O₄²⁻ each occupy 2 coordination sites. EDTA is hexadentate — it alone can satisfy a coordination number of 6.
Common Mistake
Confusing coordination number with oxidation state. Students often write “Co has coordination number 3 in [Co(NH₃)₆]Cl₃ because its oxidation state is +3.” These are completely separate things. Coordination number = number of ligand atoms directly bonded to the metal = 6 here. Oxidation state = charge on metal = +3 here. They happen to both appear in the formula but count entirely different things.
The NCERT table (Chapter 9, Class 12) showing the Werner complexes and their conductance is a standard CBSE 3-mark question. Learn that table — the logic behind it, not just by rote.