(1) Metal ion has 2 valencies: primary (= oxidation state, ionizable) and secondary (= coordination number, non-ionizable). (2) Secondary valencies have fixed spatial arrangement → geometry of complex.
-- NCERT Class 12 Chemistry, Ch. 5, p. 2Werner Theory
8 MCQs9-step worked example
Source: NCERT Coordination CompoundsPYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026
Lesson
The trap that costs marks here: confusing primary valency (oxidation state, ionizable) with secondary valency (coordination number, non-ionizable, directional). Werner distinguished these in 1893, and NEET questions exploit exactly this confusion — they give you a formula like CoCl₃·6NH₃ and ask how many ions it produces or what the coordination number is.
Werner's key postulates (NCERT Class 12 Chemistry Chapter 5, page 2):
- Metals in coordination compounds show two types of valency: primary (ionizable, satisfied by negative ions) and secondary (non-ionizable, satisfied by ligands in the coordination sphere).
- Secondary valency = coordination number. It is fixed for a given metal in a given oxidation state. For Co³⁺, this is almost always 6.
- Secondary valencies are directional — they determine the geometry (octahedral for CN 6, square planar or tetrahedral for CN 4).
How to read a formula: In [Co(NH₃)₆]Cl₃, everything inside the bracket is the coordination sphere (secondary valency = 6 NH₃ ligands). The 3 Cl⁻ outside are counter-ions satisfying primary valency. Conductivity measurements confirm 4 ions in solution: [Co(NH₃)₆]³⁺ + 3 Cl⁻.
Watch-out for NEET: When a species moves from outside to inside the bracket (compare CoCl₃·6NH₃ vs CoCl₃·5NH₃·H₂O vs CoCl₃·4NH₃), the number of ionizable Cl⁻ drops — and so does the number of ions produced. Questions test whether you can correctly partition species between the coordination sphere and the ionizable sphere.
Practice MCQs
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
According to Werner's theory, the secondary valency of a metal ion in a coordination compound corresponds to:
MCQ 2Easy RecallPractice
Werner's theory states that secondary valencies are:
MCQ 3Direct ApplicationPractice
In the compound CoCl₃·5NH₃, the coordination number of cobalt and the number of ions produced in solution are, respectively:
MCQ 4Direct ApplicationPractice
The complex [Co(NH₃)₄Cl₂]Cl has how many ionizable chloride ions and what is the coordination number of Co?
MCQ 5Easy RecallPractice
Which of the following is NOT a postulate of Werner's theory?
MCQ 6Direct ApplicationPractice
Among CoCl₃·6NH₃, CoCl₃·5NH₃, and CoCl₃·4NH₃, which produces the fewest ions in aqueous solution?
MCQ 7Concept TrapPractice
Werner proposed that secondary valencies are directional. This postulate directly explains which property of coordination compounds?
MCQ 8CalculationPractice
A coordination compound of formula CrCl₃·6H₂O has a coordination number of 6. When treated with excess AgNO₃, one mole of this compound gives 3 moles of AgCl precipitate. What is the correct structural formula?
Worked Example
Pattern: Werner coordination number — identify primary and secondary valencies from formula (P.CHE.U12.WERNER_COORDINATION_NUMBER)
- 1
Given
A cobalt(III) compound has the empirical formula CoCl₃·4NH₃. Conductivity measurements show it produces 2 ions per formula unit in solution.
- 2
Required
Write the structural formula showing the coordination sphere and determine the coordination number.
- 3
Concept
Werner's theory: species inside the coordination sphere satisfy secondary valency (non-ionizable); species outside satisfy primary valency (ionizable). Number of ions = 1 (complex ion) + number of ionizable counter-ions.
- 4
Formula / Rule
Number of ions in solution = 1 + (number of ions outside the bracket) Coordination number = total number of ligand donor atoms in the bracket.
- 5
Substitution
2 ions total → 1 complex ion + 1 counter-ion outside. Co(III) needs 3 Cl⁻ for primary valency. Only 1 Cl⁻ is outside → 2 Cl⁻ must be inside the coordination sphere as ligands.
- 6
Calculation
Inside bracket: 4 NH₃ + 2 Cl⁻ = 6 donor positions. Outside bracket: 1 Cl⁻ (ionizable). Structural formula: [CoCl₂(NH₃)₄]Cl.
- 7
Final answer
**[CoCl₂(NH₃)₄]Cl**, coordination number = **6**. Note: The integer counts (4, 2, 1, 6) are exact counting numbers and do not contribute to any significant-figure consideration.
- 8
Common trap
Counting counter-ions as ligands: if you include the 1 Cl⁻ outside the bracket as a ligand, you'd wrongly get CN = 7. Only species inside the coordination sphere count toward CN.
- 9
Similar NEET-style question
"The compound PtCl₄·2NH₃ is non-electrolyte (zero ions in solution). Write its structural formula and determine the coordination number of Pt." (Answer: [PtCl₄(NH₃)₂], CN = 6, all Cl inside, zero ionizable ions.) ---
Before solving, remember these
Formulas
Crystal field splitting (octahedral vs tetrahedral)
Tetrahedral splitting is smaller than octahedral due to fewer/farther ligands.
| Symbol | Quantity | SI Unit |
|---|---|---|
| Delta_o | octahedral splitting | J or eV |
| Delta_t | tetrahedral splitting | J or eV |
Valid when
- Same metal and same ligand
- Mostly high-spin tetrahedral due to small Δ_t
Magnetic moment of coordination complex
Same spin-only formula but n depends on high-spin/low-spin from CFT.
| Symbol | Quantity | SI Unit |
|---|---|---|
| n | unpaired electrons | - |
Valid when
- High vs low spin determined by Δ_o vs P
- Octahedral (or tetrahedral with Δ_t)
Exam Traps & Common Mistakes
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Student defaults to one spin state. Strong-field ligand (CN⁻, CO, NH₃ for some) → low-spin (Δ > P, electrons pair). Weak-field (F⁻, H₂O, Cl⁻) → high-spin.
When it triggers
Coordination compound with given ligand asking for magnetic moment, color, or spin state.
How to avoid
Memorise spectrochemical series: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO. NH₃, CN⁻, CO usually strong-field. F⁻, H₂O, Cl⁻ usually weak-field.
Root cause: concept gap
Correction
Strong-field ligand (CN⁻, CO, NH₃ when applicable): Δ > P → low-spin. Weak-field (F⁻, Cl⁻, H₂O): Δ < P → high-spin. Use spectrochemical series.
Past Year Questions
11 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
NEET 2025
1C < A < D < B
2B < D < A < C
3B < A < D < C
4C < D < A < B
NTA Answer: Option 3(final)
NEET 2025
1Statement I is false but Statement II is true
2Both Statement I and Statement II are true
3Both Statement I and Statement II are false
4Statement I is true but Statement II is false
NTA Answer: Option 4(final)
NEET 2024Revised key
1B and C
2A and D
3B and E
4E and D
NTA Answer: Option 3(revised_final)
NEET 2024Revised key
1Both Statement I and Statement II are true
2Both Statement I and Statement II are false
3Statement I is true but Statement II is false
4Statement I is false but Statement II is true
NTA Answer: Option 1(revised_final)
NEET 2023
1A, B, C, D are correct
2A, B, C, E are correct
3A, C, D, E are correct
4B, C, D, E are correct
NTA Answer: Option 2(final)
NEET 2023
Homoleptic complex from the following complexes is
1Diamminechloridonitrito-N-platinum (II)
2Pentaamminecarbonatocobalt (III) chloride
3Triamminetriaquachromium (III) chloride
4Potassium trioxalatoaluminate (III)
NTA Answer: Option 4(final)
NEET 2022
Identify the incorrect statement from the following.
1The shapes of d xy , d yz and d zx orbitals are similar to each other; and d x2−y2 and d z2 are similar to each other.
2All the five 5d orbitals are different in size when compared to the respective 4d orbitals.
3All the five 4d orbitals have shapes similar to the respective 3d orbitals.
4In an atom, all the five 3d orbitals are equal in energy in free state.
NTA Answer: Option 1(final)
NEET 2022
The IUPAC name of the complex- [Ag(H O) ][Ag(CN) ] is: 2 2 2
1diaquasilver(I) dicyanidoargentate(I)
2dicyanidosilver(II) diaquaargentate(II)
3diaquasilver(II) dicyanidoargentate(II)
4dicyanidosilver(I) diaquaargentate(I)
NTA Answer: Option 1(final)
NEET 2022
1(B) > (A) > (C)
2(A) > (B) > (C)
3(C) > (B) > (A)
4(C) > (A) > (B)
NTA Answer: Option 4(final)
NEET 2021
12-methylbutan-2-ol
22-methylpropan-2-ol
3pentan-2-ol
4pentan-3-ol
NTA Answer: Option 1(final)
NEET 2021
Ethylene diaminetetraacetate (EDTA) ion is :
1Tridentate ligand with three "N" donor atoms
2Hexadentate ligand with four "O" and two "N" donor atoms
3Unidentate ligand
4Bidentate ligand with two "N" donor atoms
NTA Answer: Option 2(final)
How NEET usually asks this
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Determine high-spin or low-spin from ligand field strength. Strong-field (CN, CO): low-spin. Weak-field (F, H2O): high-spin.
InterpretationMedium
Common distractors
forgets weak vs strong ligand
Default low-spin always
Apply IUPAC rules: ligands alphabetical, prefixes for multiplicity, oxidation state of metal.
RecallMedium
Common distractors
wrong order ligands
Uses non-alphabetical order
Identify primary and secondary valencies, coordination number from formula. Primary = ionizable ions; secondary = ligands.
InterpretationMedium
Common distractors
counts counter ions as ligands
Treats all attached species as ligands
Sources
NCERT refs: Class 12 Chemistry Chapter 5, p.2
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