Q.Give evidence that [Co(NH3)5Cl]SO4 and [Co(NH3)5(SO4)]Cl are ionisation isomers.
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Werner Coordination Theory: The Idea That Changed Inorganic Chemistry
Imagine you're looking at a salt like cobalt(III) chloride. The formula is written as CoClX3, and when you dissolve it in water, you expect to find CoX3+ and ClX− ions. But something strange happens: when you add silver nitrate (which precipitates chloride ions), only some of the chlorine comes out as silver chloride. Not all of it. And the amount that precipitates depends on how you made the compound.
This was the puzzle that faced chemists in the late 1800s. Compounds like CoClX3⋅6NHX3 (orange-yellow) and CoClX3⋅5NHX3 (purple) had the same metal and the same ligands (ammonia), but different colours, different conductivities in solution, and different numbers of chloride ions that could be precipitated. The old ideas of fixed valency couldn't explain it.
Alfred Werner proposed a radical solution in 1893. He said: a metal ion has two kinds of valency.
The Core Intuition
Think of a metal ion like a king in a castle. The king has two types of relationships:
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Primary valency (today: oxidation state) — this is the king's royal authority. It's fixed, non-directional, and satisfied by negative ions. For cobalt(III), this is +3. It's like the king's crown: it doesn't change.
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Secondary valency (today: coordination number) — this is the king's personal bodyguard. The king can have a fixed number of guards (usually 4 or 6) who stand in specific positions around him. These guards can be neutral molecules (like ammonia) or negative ions (like chloride). The key: these guards are directly attached to the metal, forming a stable cluster called the coordination sphere.
The revolutionary idea: the chloride ions that act as bodyguards (inside the coordination sphere) do not behave like free ions. They don't precipitate with silver nitrate. They don't conduct electricity. They are "locked" to the metal.
The Precise Statement
Werner Coordination Theory (1893)
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Every metal atom has two types of valency:
- Primary valency (ionisable): corresponds to the oxidation state. It is satisfied by negative ions. These ions are outside the coordination sphere and behave as free ions in solution.
- Secondary valency (non-ionisable): corresponds to the coordination number. It is satisfied by neutral molecules or negative ions directly bonded to the metal. These are inside the coordination sphere and do not dissociate.
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The secondary valencies are directional — they point to fixed positions in space around the metal, giving the complex a definite geometry (e.g., octahedral for coordination number 6, square planar for 4).
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The primary valency is non-directional — it is just a number, not a spatial arrangement.
How It Explains the Puzzle
Take the compound CoClX3⋅6NHX3 (orange-yellow). Werner said:
- Cobalt has primary valency +3 (needs three negative charges to satisfy it).
- Cobalt has secondary valency 6 (can hold six ligands around it).
- The six ammonia molecules satisfy all six secondary valencies. So the chloride ions cannot be inside the coordination sphere — they must be outside, as free ions.
- Structure: [Co(NHX3)X6]ClX3. All three chlorides precipitate with AgNOX3.
Now take CoClX3⋅5NHX3 (purple):
- Again, primary valency +3, secondary valency 6.
- Five ammonia molecules satisfy five secondary valencies. One chloride ion must fill the sixth spot — it becomes a ligand inside the sphere.
- The other two chlorides are outside as free ions.
- Structure: [Co(NHX3)X5Cl]ClX2. Only two chlorides precipitate.
The number of free ions in solution determines the conductivity and the number of precipitable chlorides. Werner's theory predicted exactly these numbers — and experiments confirmed them.
The Geometry Insight …
Why this formula?
Werner Coordination Theory: Why the Key Formulas Hold
Werner Coordination Theory (1893) revolutionized inorganic chemistry by explaining how metal ions bind ligands. Let's build the reasoning from first principles — not just memorize formulas.
1. The Core Observation: Primary vs. Secondary Valence
Werner noticed that metal compounds had two types of bonding capacity:
- Primary valence (now oxidation state): Satisfies the metal's charge — ionic in nature.
- Secondary valence (now coordination number): Determines how many ligands attach — directional, spatial in nature.
Why this distinction?
Consider CoClX3 ⋅6NHX3 (one of Werner's classic compounds).
- The compound is electrically neutral overall.
- Adding AgNOX3 precipitates all 3 Cl⁻ as AgCl — meaning all chlorides are free ions.
- Therefore, the NHX3 molecules must be directly bonded to Co, not the chlorides.
This forces the idea: Co has a fixed capacity for direct ligand attachment (secondary valence = 6 here), separate from its charge balance (primary valence = +3).
2. The Key Formula: Coordination Number = Number of Ligands Attached
Formula:
Coordination number=number of donor atoms directly bonded to the metal
Why this holds:
- Werner's experiments showed that only a fixed number of ligands could be replaced without breaking the compound's identity.
- For CoClX3 ⋅6NHX3, adding acid doesn't remove NHX3 easily — they are coordinated.
- The maximum number of such tightly bound ligands is the coordination number — a property of the metal ion, not the counterions.
Derivation from data:
If you have [Co(NHX3)X6]ClX3, conductivity measurements show 4 ions in solution ([Co(NHX3)X6]X3+ + 3 Cl⁻).
If you had [Co(NHX3)X5Cl]ClX2, conductivity shows 3 ions.
The number of chlorides inside the coordination sphere (non-precipitable) plus those outside must sum to the total chlorides. This gives the coordination number directly.
3. The Geometry Formula: Coordination Number Determines Shape
Werner proposed that secondary valences are directed in space — leading to specific geometries.
| Coordination Number | Geometry | Why? |
|---|---|---|
| 2 | Linear | Minimizes repulsion between 2 ligands |
| 4 | Tetrahedral or Square planar | 4 points in space — two arrangements possible |
| 6 | Octahedral | 6 ligands at 90° angles — most symmetric |
Why octahedral for 6?
- 6 ligands around a central atom must be placed to maximize separation.
- The octahedron (6 vertices, all equidistant from center, 90° between adjacent bonds) is the only regular polyhedron with 6 vertices.
- This explains why [Co(NHX3)X6]X3+ is octahedral — no other arrangement gives equal bond angles and distances.
4. The Isomer Counting Formula: Why 2n or n! Appears
Werner used isomer counts to confirm geometry. For an octahedral complex [MaX2bX2cX2]:
Number of geometrical isomers = 5 (not 6, not 4)
Why this formula?
- Place the two 'a' ligands: they can be cis (90°) or trans (180°).
- For each, place 'b' and 'c' in remaining positions — but symmetry reduces duplicates. …
Concept: Werner Coordination Theory — ionisation isomers exchange a ligand with the counter-ion, producing different ions in solution.
Reasoning:
- In [Co(NH3)5Cl]SO4, the chloride is coordinated to cobalt and sulphate is the free counter-ion. Dissolved in water, it gives SO42− ions, which precipitate with Ba2+ as white BaSO4.
- In [Co(NH3)5(SO4)]Cl, sulphate is coordinated and chloride is free. This isomer gives Cl− ions in solution, which precipitate with Ag+ as white AgCl. …
Ionisation isomers exchange a ligand inside the coordination sphere with an ion outside, producing different ions in solution — here, one isomer gives ClX− and the other gives SOX4X2− as the free ion, confirmed by precipitation tests.
The core idea: Werner’s coordination theory
Alfred Werner showed that in coordination compounds, a metal ion is surrounded by a fixed number of ligands in a primary (coordination) sphere. Ions outside this sphere are free to dissociate in solution. Ionisation isomers arise when the same set of atoms can be arranged so that a different ion is inside the coordination sphere versus outside. The two isomers have the same molecular formula but produce different ions when dissolved.
For the pair [Co(NHX3)X5Cl]SOX4 and [Co(NHX3)X5(SOX4)]Cl, the difference is simple: in the first, chloride is coordinated and sulfate is free; in the second, sulfate is coordinated and chloride is free. This swapping changes the electrical conductivity and, more importantly, the identity of the precipitate formed with appropriate reagents.
Step-by-step reasoning
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Identify the coordination sphere in each isomer
In [Co(NHX3)X5Cl]SOX4, the square brackets enclose the coordination sphere: CoX3+ is bonded to five NHX3 molecules and one ClX− ligand. The sulfate ion SOX4X2− lies outside, as a counterion.
In [Co(NHX3)X5(SOX4)]Cl, the sphere contains CoX3+ with five NHX3 and one SOX4X2− ligand. Now chloride ClX− is the free counterion.
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What happens when each isomer dissolves in water?
The free ions dissociate completely.
- Isomer A: [Co(NHX3)X5Cl]SOX4[Co(NHX3)X5Cl]X2++SOX4X2−
- Isomer B: [Co(NHX3)X5(SOX4)]Cl[Co(NHX3)X5(SOX4)]X++ClX−
So the solution of isomer A contains free sulfate ions; the solution of isomer B contains free chloride ions.
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Use a precipitation test to distinguish them
Add a solution of barium chloride (BaClX2) to each.
- With isomer A: free SOX4X2− reacts with BaX2+ to form a white precipitate of BaSOX4.
- With isomer B: no free sulfate is present — the sulfate is bound inside the coordination sphere and does not react. No precipitate forms.
Now add silver nitrate (AgNOX3) to fresh samples.
- With isomer A: no free chloride — no precipitate of AgCl.
- With isomer B: free ClX− gives a white curdy precipitate of AgCl.
A common mistake is to assume that because both isomers contain chlorine and sulfur, they will give the same precipitates. But only the free ions react — coordinated ligands do not precipitate with simple reagents like AgNOX3 or BaClX2.
- Confirm the charges and conductivities …
Method: Conductivity & Precipitation Test for Ionisation Isomers
Concept: Werner’s coordination theory distinguishes between ionisable (outside coordination sphere) and non-ionisable (inside coordination sphere) groups. Ionisation isomers exchange a ligand inside the sphere with a counter-ion outside, giving different ions in solution.
Steps to prove [Co(NH3)5Cl]SO4 and [Co(NH3)5(SO4)]Cl are ionisation isomers:
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Write the dissociation equations
- Isomer A: [Co(NH3)5Cl]SO4H2O[Co(NH3)5Cl]2++SO42−
- Isomer B: [Co(NH3)5(SO4)]ClH2O[Co(NH3)5(SO4)]++Cl−
-
Conductivity measurement
- Isomer A gives 2 ions: [Co(NH3)5Cl]2+ + SO42− — a 2:2-type electrolyte (a doubly charged ion pair).
- Isomer B gives 2 ions: [Co(NH3)5(SO4)]+ + Cl− — a 1:1-type electrolyte (a singly charged ion pair).
- Both give the same number of ions, but the ionic charges differ, so their molar conductivities differ (the 2+/2− pair conducts more strongly).
-
Precipitation test with BaCl2 (test for free SO42−)
- Isomer A: White precipitate of BaSO4 forms immediately because SO42− is free. …
Common Mistakes Students Make with Ionisation Isomers (Werner's Theory)
Mistake 1: Confusing the Counter-Ion with the Ligand
The error: Students often think the sulphate (SO42−) is always a ligand or always a counter-ion. They fail to check where it appears in the formula.
Why it's wrong: In [Co(NH3)5Cl]SO4, the sulphate is outside the square bracket — it is a free counter-ion. In [Co(NH3)5(SO4)]Cl, the sulphate is inside the bracket — it is a ligand bonded to cobalt.
How to avoid: Always draw a box around the coordination sphere. Everything inside is a ligand; everything outside is a counter-ion. If the same ion appears in different positions, you likely have ionisation isomers.
Mistake 2: Thinking the Compounds Are Identical
The error: Students see the same atoms (Co, NH3, Cl, SO4) and assume the compounds are the same.
Why it's wrong: The arrangement of ions between the coordination sphere and the free counter-ion changes. This changes which ions are released in solution.
How to avoid: Write the dissociation equations:
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[Co(NH3)5Cl]SO4→[Co(NH3)5Cl]2++SO42−
(gives sulphate ions in solution)
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[Co(NH3)5(SO4)]Cl→[Co(NH3)5(SO4)]++Cl−
(gives chloride ions in solution)
Key result: Different ions are released — this is the evidence for ionisation isomerism.
Mistake 3: Forgetting to Test with a Precipitating Agent
The error: Students state the isomers are different but don't mention how to prove it experimentally.
Why it's wrong: Werner's theory requires experimental evidence. Simply writing formulas is not enough.
How to avoid: Remember the BaCl₂ test and AgNO₃ test:
| Isomer | Add BaCl₂ (tests for SO42−) | Add AgNO₃ (tests for Cl−) |
|---|---|---|
| [Co(NH3)5Cl]SO4 | White precipitate of BaSO4 | No precipitate |
| [Co(NH3)5(SO4)]Cl | No precipitate | White precipitate of AgCl |
Key result: The first isomer gives BaSO4 precipitate; the second gives AgCl precipitate. This is direct evidence of different free ions.
Mistake 4: Confusing Ionisation Isomerism with Linkage Isomerism
The error: Students think this is about how SO4 binds (through O or S) — that's linkage isomerism.
Why it's wrong: In ionisation isomerism, the position of the ion (inside vs outside the coordination sphere) changes. The bonding mode of the ligand is irrelevant here.
How to avoid:
- Ionisation isomerism: Same atoms, different free ions. …
Showing the 12 most recent of 14 on this concept.
- GSEB Higher Secondary Certificate (HSC) Examination 2026Set ANNUAL1 markMCQQ.The sum of coordination number and oxidation number of the metal M in the complex [M(en)2(C2O4)]Cl is:(a) 7(b) 8(c) 9(d) 6
›Reveal solutionSolution
Find the metal's oxidation number from overall charge balance, and its coordination number by counting donor atoms from each ligand (remembering en and oxalate are bidentate).
Complex: [M(en)2(C2O4)]Cl
Oxidation number of M: The complex ion [M(en)2(C2O4)]⁺ must carry a +1 charge to balance the one Cl⁻ counter-ion. en (ethylenediamine) is a neutral ligand (charge 0); C2O4²⁻ (oxalate) carries a −2 charge.
x + 2(0) + (−2) = +1 → x = +3
…
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.The Primary and Secondary valency of the central metal ion in K[Co(OX)2(NH3)2] complex is _____ and _____ respectively.(a) 3, 4(b) 4, 3(c) 6, 3(d) 3, 6
›Reveal solutionSolution
Primary valency is the oxidation state of the metal ion; secondary valency is the coordination number - both are computed from the formula K[Co(OX)2(NH3)2].
Complex: K[Co(C2O4)2(NH3)2]. The complex ion is [Co(C2O4)2(NH3)2]-, since one K+ balances a single negative charge.
Finding the oxidation state (primary valency) of Co: oxalate (C2O4^2-) contributes -2 charge each (x2 ligands = -4), NH3 is neutral (x2 = 0). Let oxidation state of Co = x.
x + (-4) + 0 = -1 (overall charge of the complex ion)
x = +3
So primary valency = 3.
…
- GUJCET 2024Set 131 markMCQQ.In the complex K[Cr(H2O)2(C2O4)2]⋅3H2O, oxidation state and co-ordination number of the central metal ion is ________ and ________. (A) +4, 4 (B) +3, 4 (C) +4, 6 (D) +3, 6
›Reveal solutionSolution
Oxalate is a −2 bidentate ligand and water is neutral; use overall charge for the oxidation state and count donor atoms for coordination number.
Concept: In K[Cr(H2O)2(C2O4)2]⋅3H2O, K+ balances a −1 complex ion. Let Cr =x:
x+2(0)+2(−2)=−1⇒x−4=−1⇒x=+3 …
- GSEB Higher Secondary Certificate (HSC) Examination 2024Set ANNUAL1 markMCQQ.Primary and Secondary valancy of Co in the complex compound [Co(en)3]Cl3 is respectively ________.(a) 2, 3(b) 3, 6(c) 3, 3(d) 4, 6
›Reveal solutionSolution
In [Co(en)3]Cl3, cobalt is in the +3 oxidation state (primary valency = 3), and it is coordinated by 3 bidentate ethylenediamine (en) ligands, giving a coordination number of 6 (secondary valency = 6).
Primary valency = ionisable valency = oxidation state of the central metal ion = +3 for Co here (balanced by the 3 Cl- counter-ions outside the coordination sphere). …
- GUJCET 2023Set 091 markMCQQ.Which of the following species is not expected to be a ligand? (A) NH4+ (B) NO (C) H2N−CH2−CH2−NH2 (D) CO
›Reveal solutionSolution
[!TLDR]
NH4+ has no lone pair to donate, so it cannot function as a ligand.
Concept
A ligand is a Lewis base that donates one or more lone pairs of electrons to a central metal ion to form a coordinate (dative) bond.
Solution
- NH4+: nitrogen's lone pair is already used to bond the fourth H (giving the +1 charge); no lone pair remains → cannot be a ligand.
- NO: has a lone pair on N (and can donate) → ligand. …
- GUJCET 2022Set 171 markMCQQ.How many numbers of mole Ions produced from aqueous solution of 1 mole Iron (III) hexacyanido Ferrate (II) complex? (A) 4 (B) 7 (C) 5 (D) 6
›Reveal solutionSolution
Fe4[Fe(CN)6]3→4Fe3++3[Fe(CN)6]4− = 7 ions.
Concept. "Iron(III) hexacyanidoferrate(II)" has the cation Fe3+ and the complex anion [Fe(CN)6]4−. Charge balance requires 4 cations (+12) for 3 anions (−12):
Fe4[Fe(CN)6]3 …
- GSEB Higher Secondary Certificate (HSC) Examination 2022Set ANNUAL1 markMCQQ.In [Co(C2O4)2(H2O)2]-, what are the primary and secondary valencies of the central metal atom, respectively?(a) 2 and 4(b) 3 and 6(c) 3 and 4(d) 1 and 6
›Reveal solutionSolution
Primary valency corresponds to the oxidation state of the metal; secondary valency corresponds to the coordination number.
In [Co(C2O4)2(H2O)2]-, oxalate (C2O4^2-) is a bidentate ligand contributing 2 donor atoms each, and there are 2 water molecules (neutral, 1 donor atom each): total donor atoms = 2×2 + 2×1 = 6 → coordination number (secondary valency) = 6. …
- GUJCET 2021Set 151 markMCQQ.Which is not act as ligand? (A) NO (B) H2NCH2CH2NH2 (C) NH4+ (D) CO
›Reveal solutionSolution
NH₄⁺ cannot donate an electron pair (nitrogen's lone pair is used in the fourth N–H bond), so it is not a ligand.
Concept: A ligand must have a lone pair to donate to the metal. NO, ethylenediamine (H₂NCH₂CH₂NH₂) and CO all have donor lone pairs. I …
- GUJCET 2021Set 151 markMCQQ.Which type of Isomerism in isomers [Co(NH3)5(SO4)]Br and [Co(NH3)5Br]SO4? (A) Linkage (B) Ionisation (C) Coordination (D) Solvate
›Reveal solutionSolution
When a counter-ion and a ligand exchange places (inside vs outside the coordination sphere), giving different ions in solution, it is ionisation isomerism.
Concept: Ionisation isomers have the same composition but differ in which group is bound to the metal and which is the free counter-ion; they therefore give different ions on dissolving.
Here:
- [Co(NH3)5(SO4)]Br → sulfate is coordinated, Br− is the counter-ion (gives Br− in solution).
- [Co(NH3)5Br]SO4 → bromide is coordinated, SO42− is the counter-ion (gives SO42− in solution). …
- GUJCET 2020Set 071 markMCQQ.The complex having highest electrical conductivity in aqueous solution under similar conditions is ______. (A) Tetra aqua dichlorido cobalt (III) chloride (B) Triaqua trichlorido cobalt (III) (C) Penta aqua chlorido cobalt (III) chloride (D) Hexa aqua cobalt (III) chloride
›Reveal solutionSolution
[Co(H2O)6]Cl3 gives the most ions (4), so highest conductivity.
Concept — number of ions vs. conductivity. Molar conductivity rises with the number of free ions produced.
- (A) [CoCl2(H2O)4]Cl→ 2 ions
- (B) [CoCl3(H2O)3]→ neutral, 0 ions …
- GUJCET 2020Set 071 markMCQQ.Which soluble complex is formed in the leaching process of Gold? (A) [Au(OH)4]2− (B) [Au(CN)4]2− (C) [Au(OH)2]− (D) [Au(CN)2]−
›Reveal solutionSolution
Cyanide leaching of gold gives the soluble [Au(CN)2]−.
Concept — cyanide (MacArthur–Forrest) leaching. 4Au+8CN−+2H2O+O2→4[Au(CN)2]−+4OH−. Gold is Au(I) with coordination …
- GUJCET 2019Set 131 markMCQQ.The primary valency and secondary valency of central metal ion and the no. of total ions produced in aqueous solution for K[Co(OX)2(NH3)2] complex respectively is___ (A) 3, 6, 1 (B) 3, 6, 2 (C) 4, 4, 2 (D) 3, 4, 2
›Reveal solutionSolution
For K[Co(ox)2(NH3)2]: primary valency = 3, secondary valency = 6, ions in solution = 2.
Concept: Primary valency = oxidation state of the metal; secondary valency = coordination number; ionisation gives the counter-ion plus the complex ion.
Steps:
- Charge balance: complex ion charge is −1 (to balance K+). With oxalate =−2 each and NH3 neutral: x+2(−2)+0=−1⇒x=+3. Primary valency =3. …
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