Q.How many ions are produced from the complex Co(NH3)6Cl2 in solution?
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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. …
The key idea is Werner’s coordination theory: in a complex, some groups are directly bonded to the metal (in the coordination sphere) and do not dissociate in solution, while others are outside the sphere and ionize.
Reasoning:
- The formula Co(NH3)6Cl2 suggests cobalt is the central atom. Ammonia (NH3) is a neutral ligand, and chlorine can be either inside or outside the coordination sphere. …
Co(NH3)6Cl2=[Co(NH3)6]Cl2 ionises into one [Co(NH3)6]2+ cation and two Cl− ions -> 3 ions. Correct option: (iii).
The six NH3 are neutral ligands bound inside the coordination sphere and do not ionise. Both Cl− lie outside the sphere as counter ions, so cobalt is in the +2 state and the compound dissociates as: …
Method: Werner’s Coordination Theory — Primary & Secondary Valency
Step 1 — Identify the central metal and its oxidation state
- Central metal: Cobalt (Co)
- Ligands: 6 ammonia (NH3) molecules — neutral, so contribute 0 charge.
- Counter ions: 2 chloride (Cl−) ions — each has charge −1.
Let oxidation state of Co be x:
x+6(0)+2(−1)=0⇒x=+2
Step 2 — Apply Werner’s concept of valencies
- Primary valency (ionisable valency) = oxidation state = +2 → satisfied by 2 chloride ions.
- Secondary valency (coordination number) = 6 → satisfied by 6 NH3 molecules.
Step 3 — Determine the coordination sphere
- Secondary valency of 6 is filled by 6 neutral NH3 ligands. …
Common Mistakes on Werner Coordination Theory: Counting Ions from Co(NH3)6Cl2
✗ Mistake 1: Counting all atoms as separate ions
What students do: They see 6 N, 18 H, 2 Cl, and 1 Co — and think all dissociate into ions.
Why it's wrong: In coordination compounds, ligands (NH3) are covalently bonded to the central metal ion. They do not dissociate in solution. Only the counter ions outside the coordination sphere ionize.
How to avoid: Always identify the coordination sphere first. Write the formula with brackets: [Co(NH3)6]Cl2. The square brackets enclose the complex ion that stays intact.
✗ Mistake 2: Forgetting to determine the charge on the complex ion
What students do: They assume Co(NH3)6Cl2 is neutral and produces 2 Cl⁻ ions, so answer = 2.
Why it's wrong: The complex is neutral overall, but the complex ion [Co(NH3)6]2+ carries a charge. You must find that charge first.
How to avoid: Use oxidation states:
- NH3 is neutral (0 charge)
- Each Cl⁻ outside sphere has charge −1
- Let oxidation state of Co be x
- Total charge: x+6(0)+2(−1)=0⇒x=+2
So the complex ion is [Co(NH3)6]2+.
✗ Mistake 3: Counting the complex ion as multiple ions
What students do: They think [Co(NH3)6]2+ breaks into Co²⁺ and 6 NH₃ molecules.
Why it's wrong: The coordination sphere remains intact in solution. Werner's theory states that ligands are directly attached to the metal and do not dissociate.
How to avoid: Remember: primary valency (ionic bonds) gives counter ions; secondary valency (coordinate bonds) gives ligands that stay attached.
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- KEAM 2026Set eng-2026-04174 marksMCQQ.Which of the following complex has the lowest molar conductivity? (A) Dichlorotetrammineplatinum(IV) chloride (B) Dichlorotetramminecobalt(III) chloride (C) Potassium hexacyanidoferrate(II) (D) Hexaaquochromium(III) chloride (E) Pentacarbonyliron(0)
›Reveal solutionSolution
[Fe(CO)5] is a neutral molecule that gives no ions in solution, so it has essentially zero (the lowest) molar conductivity of the set.
Molar conductivity of a coordination compound rises with the number of ions it produces on dissolving:
- (A) [PtCl2(NH3)4]Cl2→ 3 ions
- (B) [CoCl2(NH3)4]Cl→ 2 ions
- (C) K4[Fe(CN)6]→ 5 ions
- (D) [Cr(H2O)6]Cl3→ 4 ions
- (E) [Fe(CO)5]→ neutral, 0 ions …
- KEAM 2026Set pha-2026-0419F4 marksMCQQ.Which of the following is a didentate ligand? (A) Ethane-1, 2-diamine (B) Chloro (C) Cyanido (D) Ammine (E) EDTA
›Reveal solutionSolution
Ethane-1,2-diamine (H2N−CH2−CH2−NH2) has two donor N atoms, so it binds the metal at two sites (bidentate/didentate). …
- KEAM 2025Set eng-2025-04234 marksMCQQ.Which of the following is a heteroleptic complex? (A) [Co(NH3)6]3+ (B) [Fe(CN)6]4− (C) [Co(SCN)4]2− (D) [Co(NH3)4Cl2]+ (E) [Co(CN)6]3−
›Reveal solutionSolution
[Co(NH3)4Cl2]+ is heteroleptic because it contains two different ligands.
Concept and Intuition
A homoleptic complex has only one type of ligand; a heteroleptic complex has more than one type. Scanning the options, only [Co(NH3)4Cl2]+ combines two ligands (NH3 and Cl^-).
Step-by-Step Solution
- [Co(NH3)6]^3+ → only NH3 (homoleptic).
- [Fe(CN)6]^4- → only CN^- (homoleptic).
- [Co(SCN)4]^2- → only SCN^- (homoleptic).
- [Co(CN)6]^3- → only CN^- (homoleptic). …
- KEAM 2025Set eng-2025-04274 marksMCQQ.Which of the following complex has the least conductivity? (A) [Co(NH3)5Cl]Cl2 (B) Cis-[Co(NH3)4Cl2]Cl (C) [Co(NH3)6]Cl3 (D) [Co(NH3)3Cl3] (E) trans-[Co(NH3)4Cl2]Cl
›Reveal solutionSolution
Molar conductivity depends on the number of ions produced. [Co(NH3)3Cl3] dissociates into 0 ions (neutral complex), so it conducts least.
Reasoning
Count the ions each complex furnishes in solution (ions outside the coordination sphere):
- (A) [Co(NH3)5Cl]Cl2→[Co(NH3)5Cl]2++2Cl− = 3 ions
- (B) cis-[Co(NH3)4Cl2]Cl→ 2 ions
- (C) [Co(NH3)6]Cl3→[Co(NH3)6]3++3Cl− = 4 ions
- (D) [Co(NH3)3Cl3]→ neutral, 0 ions …
- KEAM 2025Set eng-2025-04274 marksMCQQ.Which one of the following is an ambidentate ligand? (A) Oxalate (B) Carbon monoxide (C) Ethylene diamine (D) Ammonia (E) Nitrite
›Reveal solutionSolution
An ambidentate ligand can bind through two different donor atoms. Nitrite binds via N (nitro, –NO2) or O (nitrito, –ONO).
Reasoning
An ambidentate ligand has two different potential donor atoms but attaches through only one at a time.
- (A) Oxalate: bidentate (two O donors), not ambidentate
- (B) Carbon monoxide: monodentate (C donor)
- (C) Ethylenediamine: bidentate chelating (two N donors)
- (D) Ammonia: monodentate (N donor) …
- KEAM 2025Set pha-2025-0424A4 marksMCQQ.When CoCl3 solution is treated with excess ammonia, a violet coloured complex is formed which conducts current. Also, it gives one mole of AgCl when treated with AgNO3. What is the chemical formula of the complex? (A) [CoCl2(NH3)4]Cl (B) [CoCl3(NH3)3] (C) [CoCl(NH3)5]Cl2 (D) [Co(NH3)6]Cl3 (E) [Co(NH3)4]Cl3
›Reveal solutionSolution
The complex is [CoCl2(NH3)4]Cl.
Only ionisable chloride outside the coordination sphere precipitates with AgNO3. Since the complex gives just one mole of AgCl, exactly one Cl− is outside; the remaining two chlorides are coordinated to cobalt (with Co3+ needing a total coordination number of 6).
[CoCl2(NH3)4]Cl;⟶;[CoCl2(NH3)4]++Cl−. …
- KEAM 2024Set eng-2024-06094 marksMCQQ.A coordination compound of cobalt acts as antipernicious anaemia factor is (A) cyanocobalamine (B) carboxypeptidase (C) [Co(NH3)6]3+ (D) haemoglobin (E) myoglobin
›Reveal solutionSolution
The anti-pernicious anaemia factor is vitamin B12 = cyanocobalamine, a Co complex.
Vitamin B12 is a coordination compound of cobalt in which the metal is bound within a corrin ring. It is known as cyanocobalamine and is the anti-pernicious anaemia factor (its deficiency causes pernicious anaemia). Haemog …
- KEAM 2022Set eng-2022-P1-A14 marksMCQQ.The overall complex dissociation equilibrium constant for [Cr(H2O)6]3+ ion is 5×10−12. The overall stability constant of the complex is (A) 2×10−11 (B) 5×1011 (C) 5×1010 (D) 2×1011 (E) 0.2×1011
›Reveal solutionSolution
Stability constant = 1/K_d = 1/(5\times10^{-12}) = 2 \times 10^{11}.
Concept and Intuition
The overall stability (formation) constant is the reciprocal of the overall dissociation (instability) constant, since formation and dissociation are the reverse of each other.
Step-by-Step Solution
- K_{stability} = 1/K_{dissociation}.
- = 1/(5 \times 10^{-12}).
- = 0.2 \times 10^{12} = 2 \times 10^{11}.
Common Mistakes …
- KEAM 2021Set eng-2021-P1-A14 marksMCQQ.In which one of the following complexes, the conductivity corresponds to 1:2 electrolyte in aqueous solution? (A) Hexaamminecobalt(III) chloride (B) Tetraamminedichlorocobalt(III) chloride (C) Pentaamminechlorocobalt(III) chloride (D) Triamminetriaquachromium(III) chloride (E) Diamminesilver(I) dicyanoargentate(I)
›Reveal solutionSolution
Pentaamminechlorocobalt(III) chloride is a 1:2 electrolyte.
Concept and Intuition
Electrolyte type is set by the ratio of the charge/number of ions produced. A 1:2 electrolyte ionises into one dipositive cation and two uninegative anions (three ions total).
Step-by-Step Solution
- [Co(NH3)6]Cl3→ 4 ions (1:3).
- [Co(NH3)4Cl2]Cl→ 2 ions (1:1).
- [Co(NH3)5Cl]Cl2→[Co(NH3)5Cl]2++2Cl− = 3 ions (1:2). ✓
- [Cr(NH3)3(H2O)3]Cl3→ 1:3. …
- KEAM 2021Set eng-2021-P1-A14 marksMCQQ.The complex ion formed when the film developed in black and white photography is washed with hypo solution is (A) [Ag2(S2O3)2]3− (B) [Ag(S2O3)2]3− (C) [Ag(S2O3)2]3+ (D) [Ag2(S2O3)2]3+ (E) [Ag(S2O3)3]3−
›Reveal solutionSolution
The complex ion formed is [Ag(S2O3)2]3−.
Concept and Intuition
Hypo (sodium thiosulphate) is the fixer in photography; it dissolves unexposed silver halide by forming a soluble dithiosulphato-argentate complex.
Step-by-Step Solution
- AgBr+2Na2S2O3→Na3[Ag(S2O3)2]+NaBr.
- Ag is +1; each S2O32− is −2; two ligands give overall 1−4=−3. …
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