Q.Explain the bonding in coordination compounds in terms of Werner's postulates.
Concept understanding — Werner Coordination Theory
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
Werner didn't just count ligands — he placed them in space. For coordination number 6, he proposed an octahedral arrangement (ligands at the six corners of an octahedron). This explained why [Co(NHX3)X4ClX2]+ exists as two different compounds (isomers): one where the two chlorides are next to each other (cis) and one where they are opposite (trans). No other geometry could produce exactly two isomers.
Werner's theory was the first to show that complexes have definite three-dimensional structures. This was decades before X-ray crystallography could confirm it directly.
What It Replaced
Before Werner, chemists thought bonding was simple: each atom had a fixed valency (like carbon always forms four bonds). They tried to write chain structures for coordination compounds (like organic molecules), but it failed — you couldn't explain why CoClX3⋅6NHX3 and CoClX3⋅5NHX3 were different compounds with the same metal and ligands.
Werner's key break: the metal can bond to more species than its oxidation state would suggest, and those bonds are not all the same type.
The Legacy
Werner won the Nobel Prize in 1913. His theory:
- Introduced the concept of coordination number and coordination sphere.
- Explained isomerism in complexes (geometric, optical).
- Laid the foundation for modern coordination chemistry, crystal field theory, and ligand field theory.
- Showed that inorganic compounds could have complex, predictable geometries — not just simple salts.
When you see a formula like [Co(NHX3)X6]ClX3, read the square brackets as "the castle walls". Everything inside is tightly bound to the metal; everything outside is free. That's Werner's idea in a nutshell.
Werner's coordination theory is the historical and conceptual foundation of the NCERT/CBSE Class 12 Chemistry chapter on Coordination Compounds, and ‘Werner's theory of coordination compounds’ is one of the most frequently asked important questions in board exams, JEE Main and NEET. Understanding primary and secondary valency as Werner defined them is essential groundwork for every other topic in this chapter.
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 count comes from combinatorial reasoning on a fixed octahedral framework, not arbitrary permutation.
General principle:
Number of isomers=symmetry factortotal arrangements
This is not a simple n! — it depends on the point group symmetry of the complex.
5. The Valence Sum Rule: Primary + Secondary = Constant?
Not a fixed sum!
Werner's theory does not say primary + secondary valence = constant.
Example:
- CoX3+ has primary valence = 3, secondary = 6 → sum = 9
- PtX4+ has primary = 4, secondary = 6 → sum = 10
Why no fixed sum?
- Primary valence depends on the metal's oxidation state (variable).
- Secondary valence depends on the metal's size and electronic configuration (also variable).
- They are independent properties — the only link is that both must be satisfied for a stable complex.
Summary: The Core Insight
Werner's formulas hold because:
- Coordination number is an experimentally determined maximum — not a theoretical guess.
- Geometry follows from minimizing ligand-ligand repulsion on a sphere.
- Isomer counts follow from symmetry constraints on a fixed polyhedron.
- Primary and secondary valences are independent — no single formula links them.
The real power of Werner's theory: it turned coordination chemistry from a list of random compounds into a predictive, spatial science — long before X-ray crystallography confirmed the geometries.
Werner’s Coordination Theory was the first successful model to explain bonding in coordination compounds. It proposed that metal ions have two types of valency: primary valency (ionisable, corresponding to oxidation state) and secondary valency (non-ionisable, corresponding to coordination number). The secondary valencies are directed in space around the metal, giving a fixed geometry.
Reasoning steps:
- Primary valency is satisfied by negative ions (e.g., Cl⁻ in [Co(NHX3)X6]ClX3), and these ions are ionisable — they precipitate with Ag⁺.
- Secondary valency is satisfied by neutral molecules or anions (e.g., NH₃ in the same complex), and these are non-ionisable — they remain bound to the metal even in solution.
- The number of secondary valencies (coordination number) is fixed for a given metal, and they are arranged in a definite stereochemistry (e.g., octahedral for Co³⁺, square planar for Pt²⁺).
Werner’s postulates explain bonding by distinguishing primary (ionisable, corresponding to oxidation state) and secondary (non-ionisable, satisfied by ligands) valencies, with the secondary valencies directed in space to give a fixed geometry and stoichiometry.
Werner’s coordination theory explains bonding in coordination compounds by proposing that metal ions have two types of valencies — primary (ionisable) and secondary (non-ionisable) — and that ligands occupy fixed positions in space around the metal, giving a definite geometry.
Werner’s theory was revolutionary because it moved beyond simple ionic or covalent bonding ideas. Before Werner, chemists struggled to explain why compounds like CoClX3⋅6NHX3 (which we now call [Co(NHX3)X6]ClX3) did not behave like a simple mixture of CoClX3 and NHX3. Werner proposed that the metal ion has two distinct kinds of bonding capacity.
Primary valency corresponds to the oxidation state of the metal — it is satisfied by negative ions and is non-directional. Secondary valency corresponds to the coordination number — it is satisfied by neutral molecules or negative ions (ligands) and is directional, pointing to fixed positions in space around the metal. The secondary valencies give the compound its geometry.
Let’s see how this applies step by step.
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Identify the central metal and its primary valency.
In [Co(NHX3)X6]ClX3, the central atom is cobalt. The primary valency of Co is 3 (since three ClX− ions are needed to neutralise the charge). This is the oxidation state of Co: +3.
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Determine the secondary valency (coordination number).
Six NHX3 molecules are directly attached to Co — these satisfy the secondary valency. So the coordination number is 6. Werner said secondary valencies are always satisfied by ligands, and they are fixed in number for a given metal ion.
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Assign the geometry based on secondary valencies.
For coordination number 6, Werner correctly predicted an octahedral arrangement. The six ligands occupy the six corners of an octahedron around the metal. This explained why [Co(NHX3)X6]ClX3 does not show isomerism due to different ligand positions — all six positions are equivalent.
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Distinguish between ionisable and non-ionisable groups.
The three ClX− ions satisfy the primary valency and are ionisable — they precipitate as AgCl when treated with AgNOX3. The six NHX3 molecules satisfy secondary valencies and are non-ionisable — they do not precipitate. This matched experimental conductivity and precipitation data perfectly.
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Explain the bonding in other compounds using the same logic.
For example, [Co(NHX3)X5Cl]ClX2:
- Primary valency of Co = 3 (two ClX− ions outside + one ClX− inside).
- Secondary valency = 6 (five NHX3 + one Cl).
- Only two ClX− are ionisable (precipitate with AgNOX3), confirming the third Cl is bonded directly to Co via secondary valency.
A common mistake is to think that primary valency equals the number of ligands. It does not — primary valency is the oxidation state, while secondary valency is the coordination number. They are independent.
Werner’s theory is essentially the first successful model of coordination compounds. It correctly predicted the existence of isomers (like geometrical isomers in [Co(NHX3)X4ClX2]X+) long before X-ray crystallography confirmed them.
Werner’s postulates state that metal ions possess primary (ionisable, non-directional) and secondary (non-ionisable, directional) valencies, and that secondary valencies determine the geometry — for example, in [Co(NHX3)X6]ClX3, Co has primary valency 3 and secondary valency 6, giving an octahedral structure.
Werner Coordination Theory — Bonding Explanation
Method: Werner's Postulate Approach
This method explains bonding in coordination compounds using the primary valency and secondary valency concepts proposed by Alfred Werner in 1893.
Step 1 — Identify the Central Metal Atom
- The metal atom (usually a transition metal) acts as the central atom.
- Example: In [Co(NHX3)X6]ClX3, the central atom is cobalt (Co).
Step 2 — Assign Primary Valency (Ionisable Valency)
- Primary valency corresponds to the oxidation state of the metal.
- It is satisfied by negative ions (anions) and is non-directional.
- It is written outside the coordination sphere (square brackets).
Example:
In [Co(NHX3)X6]ClX3, Co has primary valency = +3 (since three ClX− ions are outside).
Step 3 — Assign Secondary Valency (Coordination Number)
- Secondary valency corresponds to the coordination number of the metal.
- It is satisfied by neutral molecules or negative ions (ligands) inside the coordination sphere.
- It is directional and determines the geometry of the complex.
Example:
In [Co(NHX3)X6]ClX3, Co has secondary valency = 6 (six NHX3 ligands).
Step 4 — Determine Geometry from Secondary Valency
- Secondary valency fixes the spatial arrangement of ligands around the metal.
| Coordination Number | Geometry |
|---|---|
| 2 | Linear |
| 4 | Tetrahedral or Square planar |
| 6 | Octahedral |
Example:
[Co(NHX3)X6]X3+ has octahedral geometry (secondary valency = 6).
Step 5 — Distinguish Between Ionisable and Non-Ionisable Groups
- Primary valency groups are outside the bracket — they are ionisable (precipitate with suitable reagents).
- Secondary valency groups are inside the bracket — they are non-ionisable (do not precipitate).
Example:
[Co(NHX3)X6]ClX3 gives 3 moles of AgCl with AgNOX3 (all three ClX− are ionisable).
[Co(NHX3)X5Cl]ClX2 gives only 2 moles of AgCl (one ClX− is inside the sphere, non-ionisable).
Step 6 — Summarise Bonding in Terms of Postulates
| Werner's Postulate | Explanation |
|---|---|
| 1. Every metal has two types of valencies | Primary (oxidation state) and secondary (coordination number) |
| 2. Secondary valencies are directional | They determine geometry (e.g., octahedral, tetrahedral) |
| 3. Primary valencies are satisfied by anions | They are ionisable and written outside the coordination sphere |
| 4. Secondary valencies are satisfied by ligands | They are non-ionisable and written inside the coordination sphere |
Final Key Takeaway
Werner's theory explains bonding by separating the metal's oxidation state (primary valency) from its coordination number (secondary valency), with the latter dictating the complex's shape and the former determining its charge and ionisable groups.
This method is concept-first: understand why the complex has a certain formula and geometry, then apply to any given coordination compound.
Common Mistakes in Werner's Coordination Theory (and How to Avoid Them)
Werner's theory is the foundation of coordination chemistry, but students often slip on a few key points. Here are the most frequent errors and how to fix them.
Mistake 1: Confusing Primary Valency with Secondary Valency
The error: Students think primary valency is the total charge on the complex, or that secondary valency is the oxidation state.
The truth:
- Primary valency = oxidation state of the central metal ion (ionizable, satisfied by anions)
- Secondary valency = coordination number (non-ionizable, satisfied by ligands, directional)
How to avoid: Memorise the distinction with a simple example:
- In [Co(NHX3)X6]ClX3, primary valency of Co = +3 (satisfied by 3 Cl⁻ ions), secondary valency = 6 (satisfied by 6 NH₃ molecules).
Mistake 2: Forgetting That Secondary Valency Is Fixed and Directional
The error: Students treat secondary valency as variable or non-geometric.
The truth: Werner proposed that secondary valencies are fixed in number for a given metal and point to fixed positions in space — this is the origin of stereochemistry (octahedral, square planar, tetrahedral).
How to avoid: Always draw the geometry when explaining. For example, [Co(NHX3)X6]X3+ is octahedral — all six positions are equivalent.
Mistake 3: Mixing Up Ionizable vs. Non-ionizable Groups
The error: Students think all anions satisfy primary valency, or that all neutral molecules satisfy secondary valency.
The truth:
- Primary valency is satisfied by anions (Cl⁻, SO₄²⁻, etc.) — these are ionizable and precipitate with Ag⁺, Ba²⁺, etc.
- Secondary valency can be satisfied by neutral molecules (NH₃, H₂O) or anions (Cl⁻, CN⁻) — these are non-ionizable and do not precipitate.
Example: In [Co(NHX3)X5Cl]ClX2:
- One Cl⁻ satisfies secondary valency (inside coordination sphere) — does not precipitate with Ag⁺
- Two Cl⁻ satisfy primary valency (outside sphere) — precipitate with Ag⁺
How to avoid: Practise writing the complex formula with square brackets — everything inside is secondary valency, everything outside is primary.
Mistake 4: Thinking Werner Explained All Bonding (Covalent/Electrostatic)
The error: Students believe Werner's theory describes the nature of the metal-ligand bond.
The truth: Werner's theory is purely structural — it explains how many and where ligands attach, but not why (that came later with VBT, CFT, MOT).
How to avoid: State clearly: "Werner's postulates describe the number and spatial arrangement of ligands, not the electronic structure of the bond."
Mistake 5: Ignoring the Existence of Isomers
The error: Students fail to connect secondary valency directionality to isomerism.
The truth: Because secondary valencies have fixed positions, complexes can show geometrical isomerism (e.g., cis/trans in [Co(NHX3)X4ClX2]X+) and optical isomerism.
How to avoid: When explaining Werner's postulates, always mention that the fixed spatial arrangement predicts isomerism — this was a major triumph of his theory.
Mistake 6: Using the Wrong Terminology in Exams
The error: Students write "primary valency = ionic bond" or "secondary valency = covalent bond."
The truth: Werner did not use the terms ionic/covalent. He said:
- Primary valency = ionizable (satisfied by anions)
- Secondary valency = non-ionizable (satisfied by ligands, directional)
How to avoid: Use Werner's own language: "ionizable" and "non-ionizable" or "satisfied by anions" and "satisfied by ligands."
Quick Revision Checklist
| Concept | Common Mistake | Correct Understanding |
|---|---|---|
| Primary valency | = charge on complex | = oxidation state of metal |
| Secondary valency | = variable | = fixed coordination number |
| Ionizable groups | All anions are ionizable | Only those outside coordination sphere |
| Bond nature | Werner explained covalent bonds | Werner explained structure, not bond type |
| Isomerism | Not linked to theory | Direct consequence of fixed geometry |
Final tip: When answering an exam question on Werner's postulates, always:
- State the two types of valency clearly.
- Give a concrete example with a formula.
- Mention that secondary valencies have fixed spatial positions (explaining isomerism).
- Prefer Werner's own terms — "ionisable"/"non-ionisable" — rather than flatly labelling the valencies as "ionic bonds" or "covalent bonds"; equating a valency with a bond type is the classic slip.
Showing the 12 most recent of 66 on this concept.
- CBSE 2026Set 56/3/11 markMCQQ.According to Werner's theory, the primary valencies of the central metal atom : (A) are satisfied by neutral molecules or negative ions. (B) are equal to its coordination number. (C) are satisfied by negative ions. (D) are non-ionisable.
›Reveal solutionSolution
Werner’s theory distinguishes primary valencies (ionisable, satisfied only by negative ions, equal to oxidation state) from secondary valencies (non-ionisable, satisfied by neutral molecules or negative ions, equal to coordination number). The correct answer is (C).
Werner’s Coordination Theory was a breakthrough because it explained why compounds like CoClX3⋅6NHX3 (which we now write as [Co(NHX3)X6]ClX3) behave so differently from simple salts. The key insight: a metal ion has two kinds of bonding capacity — not just one.
Primary valency (now called oxidation state) is the metal’s ionic bonding capacity. It is satisfied only by negative ions (anions), because it arises from the metal’s need to neutralise its positive charge. These bonds are ionisable — they break apart in solution, giving the familiar conductivity and precipitation tests.
Secondary valency (now called coordination number) is the metal’s ability to bind ligands directly. It is satisfied by neutral molecules (like NHX3) or negative ions (like ClX−). These bonds are non-ionisable — they stay attached to the metal even in solution.
Now let’s apply this to the options.
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Option (A): “Primary valencies are satisfied by neutral molecules or negative ions.”
This is false. Primary valencies are satisfied only by negative ions. Neutral molecules satisfy secondary valencies. For example, in [Co(NHX3)X6]ClX3, the three ClX− ions satisfy the primary valency (Co³⁺ needs three negative charges), while six NHX3 molecules satisfy the secondary valency.
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Option (B): “Primary valencies are equal to its coordination number.”
This is false. Primary valency equals the oxidation state of the metal. Coordination number equals the secondary valency. They are often different. In [Co(NHX3)X6]ClX3, primary valency = 3, coordination number = 6.
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Option (C): “Primary valencies are satisfied by negative ions.”
This is true. Werner explicitly stated that primary valencies are satisfied only by anions. This is the defining property.
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Option (D): “Primary valencies are non-ionisable.”
This is false. Primary valencies are ionisable — they dissociate in solution. Secondary valencies are non-ionisable. In [Co(NHX3)X6]ClX3, the three ClX− ions (primary) are free in solution and can be precipitated by AgX+, while the six NHX3 (secondary) remain bound to cobalt.
Watch outA common mistake is to confuse primary valency with coordination number. Remember: primary = oxidation state (charge neutralisation), secondary = coordination number (ligand binding). They are not the same.
TipA quick way to identify primary valency in a complex: count the number of ionisable anions outside the coordination sphere. That number equals the primary valency (oxidation state) of the metal.
✓Final answerThe correct option is (C) — primary valencies are satisfied by negative ions.
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- CBSE 2026Set 56/1/11 markMCQQ.The secondary valency of Co in the complex [Co(NH3)5(NO2)]2+ is (A) 5 (B) 1 (C) 4 (D) 6
›Reveal solutionSolution
In Werner’s coordination theory, secondary valency equals the coordination number — the number of ligand donor atoms directly bonded to the metal. For [Co(NH3)5(NO2)]2+, there are 5 NH₃ molecules (each donating one N) and one NO₂⁻ ligand (also donating one N), giving a total of 6 donor atoms. So the secondary valency is 6.
Werner’s coordination theory is the foundation here. He proposed that metal ions have two kinds of valency: primary (ionisable, corresponding to oxidation state) and secondary (non-ionisable, corresponding to coordination number). The secondary valency is simply the number of ligand donor atoms directly attached to the central metal ion — in other words, the coordination number.
For the complex [Co(NH3)5(NO2)]2+, we need to count how many atoms are actually bonded to cobalt.
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Identify the ligands and their denticity.
- NH₃ (ammine) is a monodentate ligand — it binds through the lone pair on nitrogen. Each NH₃ contributes one donor atom.
- NO₂⁻ (nitrito or nitro, depending on binding mode) is also monodentate when it binds through nitrogen (the more common nitro form) or through oxygen. In either case, it uses one donor atom per ligand. The problem doesn’t specify the linkage isomer, but that doesn’t change the count — it’s still one donor atom.
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Count the total number of donor atoms.
- Five NH₃ ligands: 5×1=5 donor atoms.
- One NO₂⁻ ligand: 1×1=1 donor atom.
- Total = 5+1=6 donor atoms.
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Relate this to secondary valency.
Werner defined secondary valency as the number of groups directly coordinated to the metal — exactly the coordination number. So here, secondary valency = 6.
Watch outA common mistake is to confuse secondary valency with the charge on the complex or the number of ligands (5 ligands, not 6). But remember: secondary valency counts donor atoms, not ligand molecules. Even though there are only 6 ligands (5 NH₃ + 1 NO₂⁻), each is monodentate, so the count matches the number of ligands here — but that’s a coincidence. If a ligand were bidentate (like ethylenediamine), the number of ligands would be smaller than the secondary valency.
TipA quick check: the coordination number of cobalt in most octahedral complexes is 6. Since NH₃ and NO₂⁻ are both strong-field ligands and Co(III) is d⁶, an octahedral geometry is expected. So even without counting, you could guess 6 — but always verify by counting donor atoms.
✓Final answerThe secondary valency of Co in [Co(NH3)5(NO2)]2+ is 6, which corresponds to option (D).
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- CBSE 2026Set DZ1 markMCQQ.What is the coordination number of Co in the complex K3[Co(C2O4)3] ?(a) 3(b) 4(c) 5(d) 6
›Reveal solutionSolution
Oxalate is a bidentate ligand; three of them occupy six coordination sites, so the coordination number of Co is 6 — option (d).
The coordination number is the number of ligand donor atoms directly bonded to the central metal ion (not the number of ligand molecules).
In K3[Co(C2O4)3] the ligand is the oxalate ion C2O42−. Each oxalate binds the metal through two oxygen atoms, so it is a bidentate (chelating) ligand.
Donor atoms=3 oxalates×2=6
Hence Co is surrounded by six O-donor atoms in an octahedral geometry.
✓Final answer(d) 6.
- CBSE 2026Set ANNUAL1 markMCQQ.What is the coordination number of Cobalt in [Co(en)3]Cl3 ?(a) 6(b) 5(c) 4(d) 3
›Reveal solutionSolution
Each 'en' ligand is bidentate, so [Co(en)3]Cl3 has coordination number 6.
In [Co(en)3]Cl3, ethylenediamine (en, H2NCH2CH2NH2) acts as a bidentate ligand — it has two nitrogen donor atoms, each with a lone pair, that can simultaneously coordinate to the central metal ion, forming a five-membered chelate ring with cobalt.
Since there are three 'en' ligands attached to the cobalt centre, and each contributes 2 donor atoms, the total number of coordinate bonds (and hence the coordination number) is:
3×2=6
The three Cl− ions are ionizable counter-ions lying outside the coordination sphere (as confirmed by the complex giving 3 moles of AgCl precipitate per mole with AgNO3) and do not count towards the coordination number.
✓Final answer(a) 6 is the coordination number of cobalt in [Co(en)3]Cl3.
- CBSE 2026Set ANNUAL1 markMCQQ.Ambidentate ligand is(a) H2O(b) NH3(c) NO2-(d) Cl-
›Reveal solutionSolution
An ambidentate ligand has two different donor atoms but binds through only one of them at a time, depending on conditions.
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H2O, NH3 and Cl- each have only one type of donor atom, so they are simple monodentate ligands, not ambidentate.
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NO2- has two possible donor atoms - N (giving a nitro complex) or O (giving a nitrito complex) - so it is the classic ambidentate ligand, along with SCN-/NCS- and CN-/NC-.
✓Final answer(c) NO2−.
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- CBSE 2026Set ANNUAL1 markMCQQ.Coordination number of Pt in [Pt(NH3)2 Cl (NO2)] complex is(a) 3(b) 4(c) 5(d) 6
›Reveal solutionSolution
Coordination number = the total number of ligand donor atoms directly bonded to the central metal atom/ion.
In [Pt(NH3)2Cl(NO2)], the ligands bonded to Pt are: 2 x NH3 + 1 x Cl- + 1 x NO2- = 4 donor atoms.
✓Final answer(b) 4.
- CBSE 2026Set ANNUAL1 markMCQQ.Identify the homoleptic complex among the following compounds:(a) [Co(NH3)4Cl2]+(b) [Ni(CN)4]2-(c) [Cr(NH3)2Cl2(en)]+(d) Both (A) and (C)
›Reveal solutionSolution
A homoleptic complex is one in which the metal is bonded to only a single kind of donor ligand; a heteroleptic complex has more than one kind.
Checking each option:
- [Co(NH3)4Cl2]+: contains two different ligands, NH3 and Cl- (heteroleptic).
- [Ni(CN)4]2-: contains only CN- ligands, all of the same type (homoleptic).
- [Cr(NH3)2Cl2(en)]+: contains NH3, Cl-, and ethylenediamine (en) - three different ligand types (heteroleptic).
Only [Ni(CN)4]2- has a single ligand type throughout.
✓Final answer(b) [Ni(CN)4]2-.
- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following is a bidentate ligand?(a) CN-(b) NH3(c) C2O4^2-(d) CO3^2-
›Reveal solutionSolution
A bidentate ligand has two donor atoms that simultaneously coordinate to the same central metal atom, forming a ring (chelate).
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CN- has one donor atom (C), so it is monodentate.
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NH3 has one donor atom (N), so it is monodentate.
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C2O4^2- (oxalate ion) has two oxygen atoms, one from each carboxylate group, that can each donate a lone pair to the metal simultaneously - this makes it bidentate, forming a five-membered chelate ring.
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CO3^2- typically binds through one or two oxygens but is generally classified as monodentate or, in rarer bridging modes, differently - it is not the standard bidentate example here.
✓Final answer(c) C2O4^2-.
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- CBSE 2026Set ANNUAL1 markMCQQ.The co-ordination number of Cr in K3[Cr(C2O4)3] is: (as printed; the paper's option values are +3/+4/+2/+6)(a) +3(b) +4(c) +2(d) +6
›Reveal solutionSolution
As printed, the options (+3,+4,+2,+6) are oxidation-state values, so this question is really testing the oxidation number of Cr, which is +3. (For completeness: the true coordination number — the count of donor atoms bonded to Cr — is 6, not among the given options, since oxalate is bidentate and there are three oxalate ligands.)
Oxidation number of Cr: In K3[Cr(C2O4)3], potassium contributes +1 each (three K+ ions), and each oxalate ion C2O42− carries a charge of −2. Let the oxidation number of Cr be x:
3(+1)+x+3(−2)=0
3+x−6=0⟹x=+3
Coordination number (for clarity): Oxalate (C2O42−) is a bidentate ligand — it binds through two oxygen donor atoms. With three oxalate ligands, Cr is bonded to 3×2=6 donor atoms, so the coordination number is 6 (octahedral geometry), not any of the printed options.
✓Final answerOxidation number of Cr is +3 (matching the printed options); the true coordination number is 6.
- CBSE 2026Set ANNUAL1 markQ.What is the oxidation number of Fe in [Fe(CN)6]4−?
›Reveal solutionSolution
Balancing the ligand charges against the overall complex-ion charge gives the oxidation state of iron.
Working it out
Let the oxidation number of Fe be x. Cyanide is an anionic ligand, CN−, contributing −1 each; there are 6 of them:
x+6(−1)=−4 (overall charge of the complex ion)
x−6=−4⟹x=+2
So in [Fe(CN)6]4− (the ferrocyanide ion), iron is present as Fe2+.
✓Final answerOxidation number of Fe =+2.
- CBSE 2026Set ANNUAL1 markQ.What is the coordination number of Fe in [Fe(EDTA)]⁻?
›Reveal solutionSolution
EDTA is a hexadentate chelating ligand (2 N + 4 O donor atoms), so a single EDTA fills all six coordination sites — coordination number of Fe = 6.
Concept. The coordination number of a metal ion is the number of ligand donor atoms directly bonded to it (the number of coordinate bonds), not the number of ligand molecules.
Reasoning. EDTA4− (ethylenediaminetetraacetate) is a hexadentate ligand: it donates through its two amine nitrogen atoms and four carboxylate oxygen atoms. Even though only one EDTA molecule is present, it forms six coordinate bonds to the central iron, wrapping around it as a chelate.
donor atoms=2N+4O=6
✓Final answerThe coordination number of Fe in [Fe(EDTA)]− is 6.
- CBSE 2026Set ANNUAL1 markMCQQ.Assertion (A): Glycinate ion H₂NCH₂COO⁻ is a chelating ligand. Reason (R): It can link through either nitrogen or anionic oxygen.(a) Both A and R are true and R is the correct explanation of A.(b) Both A and R are true but R is not the correct explanation of A.(c) A is true but R is false.(d) A is false but R is correct.
›Reveal solutionSolution
A (glycinate is chelating) is true, but the reason given describes ambidentate behaviour (binding through one OR the other), not chelation (binding through both at once), so R is false — option (C).
Assertion (A): The glycinate ion H2N−CH2−COO− is a chelating ligand. This is true: it is a bidentate ligand that binds a metal through the nitrogen of the −NH2 group and the anionic oxygen of the −COO− group at the same time, forming a stable five-membered chelate ring.
Reason (R): "It can link through either nitrogen or anionic oxygen." This describes an ambidentate ligand — one that attaches through one donor atom or the other, but only one at a time. That is not what makes a ligand chelating. A chelating ligand must use two (or more) donor atoms simultaneously to grip the same metal. So R, as worded, is a false explanation of A.
✓Final answer(C) A is true but R is false — glycinate chelates by using both N and O together, not "either/or".
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