The trap that costs marks here: confusing the coordination number with the total number of ions or atoms attached to a metal. Counter-ions sitting outside the coordination sphere are not ligands — counting them inflates the coordination number and leads straight to a wrong answer.
What a ligand actually is. A ligand is a neutral molecule or ion that donates at least one lone pair of electrons to a central metal atom or ion, forming a coordinate bond (NCERT Class 12 Chemistry Chapter 5, page 121). The metal is the Lewis acid; the ligand is the Lewis base. Only species directly bonded to the metal inside the coordination sphere count.
Denticity — how many donor atoms one ligand has. A monodentate ligand (Cl⁻, NH₃, H₂O) donates through one atom. A bidentate ligand (ethylenediamine, oxalate) donates through two atoms. Polydentate ligands like EDTA⁴⁻ donate through six atoms. Denticity matters because the coordination number equals the total number of donor atoms, not the total number of ligand molecules.
Coordination number. It is the number of ligand donor atoms directly bonded to the central metal ion. For [Co(NH₃)₆]³⁺, six monodentate NH₃ molecules give coordination number 6. For [Co(en)₃]³⁺, three bidentate ethylenediamine ligands still give coordination number 6 (3 ligands × 2 donor atoms each). The counter-ions (Cl⁻ outside the bracket in [Co(NH₃)₆]Cl₃) do not count.
Primary vs secondary valency (Werner's theory). Primary valency is the oxidation state — satisfied by counter-ions and ionizable. Secondary valency is the coordination number — satisfied by ligands and fixed for a given complex. NEET questions frequently ask you to distinguish these: a common distractor treats all attached species as ligands, conflating primary and secondary valencies.
Watch-out: When a formula like CoCl₃·6NH₃ is written without brackets, identify which species are inside the coordination sphere (ligands) and which are ionizable counter-ions before assigning the coordination number.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following is a bidentate ligand?
Answer: C. Ethylenediamine has two nitrogen donor atoms, each donating a lone pair to the metal, making it bidentate (NCERT Class 12 Chemistry Chapter 5, page 121).
Why A is wrong: A is wrong because Cl⁻ has only one donor atom — it is monodentate.
Why B is wrong: B is wrong because NH₃ donates through its single nitrogen lone pair — it is monodentate.
Why D is wrong: D is wrong because H₂O donates through one oxygen lone pair — it is monodentate.
The coordination number of the central metal ion in [PtCl₆]²⁻ is:
Answer: C. Six monodentate Cl⁻ ligands are directly bonded to Pt, so the coordination number is 6 (NCERT Class 12 Chemistry Chapter 5, page 121).
Why A is wrong: A is wrong because all six chloride ligands are inside the coordination sphere, not just two.
Why B is wrong: B is wrong because counting only four ignores two of the six Cl⁻ ligands bonded to Pt.
Why D is wrong: D is wrong because there are exactly six ligand donor atoms, not eight.
In Werner's theory, the primary valency of the metal in [Co(NH₃)₆]Cl₃ is:
Answer: A. The primary valency equals the oxidation state of Co. Three Cl⁻ ions are ionizable counter-ions, so the primary valency is +3 (NCERT Class 12 Chemistry Chapter 5, page 121).
Why B is wrong: B is wrong because 6 is the secondary valency (coordination number), not the primary valency (trap: confusing primary and secondary valencies).
Why C is wrong: C is wrong because 9 incorrectly adds primary and secondary valencies together — they are independent properties.
Why D is wrong: D is wrong because Co clearly has a +3 oxidation state balanced by the three Cl⁻ counter-ions.
The coordination number of Co³⁺ in [Co(en)₂Cl₂]⁺ is:
Answer: D. Two bidentate en ligands contribute 2 × 2 = 4 donor atoms; two monodentate Cl⁻ contribute 2 donor atoms. Total coordination number = 4 + 2 = 6 (NCERT Class 12 Chemistry Chapter 5, page 121).
Why A is wrong: A is wrong because it counts the number of ligand molecules (2 en + 2 Cl = 4) instead of the number of donor atoms — en is bidentate, so each en contributes 2 (trap: ignoring denticity when counting coordination number).
Why B is wrong: B is wrong because there is no basis for eight donor atoms in this complex.
Why C is wrong: C is wrong because 2 counts only the Cl⁻ ligands and ignores the four donor atoms from the two en ligands.
In CoCl₃·4NH₃, the number of ionizable chloride ions (counter-ions outside the coordination sphere) is 1. What is the coordination number of Co³⁺?
Answer: B. If 1 Cl⁻ is ionizable, then 2 Cl⁻ are inside the coordination sphere as ligands. Four NH₃ are also ligands. All are monodentate, so coordination number = 2 + 4 = 6. The complex is [CoCl₂(NH₃)₄]Cl (NCERT Class 12 Chemistry Chapter 5, page 121).
Why A is wrong: A is wrong because it counts only the four NH₃ molecules and ignores the two Cl⁻ inside the coordination sphere (trap: counting counter-ions as non-ligands but then also excluding coordinated Cl⁻).
Why C is wrong: C is wrong because it likely miscounts by including only one Cl⁻ ligand instead of two — if 1 of 3 is ionizable, 2 remain coordinated.
Why D is wrong: D is wrong because it incorrectly includes all 3 Cl⁻ plus all 4 NH₃ — one Cl⁻ is a counter-ion and does not contribute to the coordination number.
EDTA⁴⁻ is a hexadentate ligand. What is the coordination number of Fe³⁺ in [Fe(EDTA)]⁻?
Answer: A. One EDTA⁴⁻ ligand donates through 6 donor atoms (2 nitrogen + 4 oxygen), so the coordination number is 6 (NCERT Class 12 Chemistry Chapter 5, page 121).
Why B is wrong: B is wrong because 4 might come from counting only the carboxylate oxygens and ignoring the two nitrogen donors.
Why C is wrong: C is wrong because 1 is the number of ligand molecules, not the number of donor atoms — coordination number counts donor atoms (trap: confusing number of ligands with coordination number).
Why D is wrong: D is wrong because EDTA has exactly 6 donor atoms, not 8.
The compound CoCl₃·6NH₃ gives 3 moles of AgCl on treatment with excess AgNO₃. Which of the following correctly represents the complex?
Answer: D. Three moles of AgCl means all 3 Cl⁻ are ionizable counter-ions outside the coordination sphere. Therefore all 6 NH₃ are ligands, giving [Co(NH₃)₆]Cl₃ with coordination number 6 (NCERT Class 12 Chemistry Chapter 5, page 121).
Why A is wrong: A is wrong because it places all three Cl⁻ inside the coordination sphere with zero ionizable — this would give no precipitate with AgNO₃.
Why B is wrong: B is wrong because it places one Cl⁻ inside the coordination sphere, leaving only 2 ionizable — that would give only 2 moles of AgCl, not 3 (trap: miscounting ionizable vs coordinated chlorides).
Why C is wrong: C is wrong because it places two Cl⁻ inside the coordination sphere, leaving only 1 ionizable — that gives only 1 mole of AgCl.
A complex of Cr³⁺ with formula CrCl₃·5NH₃ produces 2 moles of AgCl per mole of complex with excess AgNO₃. What is the coordination number of Cr³⁺ in this complex?
Answer: B. Step 1: 2 moles AgCl means 2 Cl⁻ are ionizable counter-ions, so 1 Cl⁻ is inside the coordination sphere. Step 2: Ligands = 5 NH₃ (monodentate, 5 donor atoms) + 1 Cl⁻ (monodentate, 1 donor atom) = 6 donor atoms total. Coordination number = 6. The complex is [CrCl(NH₃)₅]Cl₂ (NCERT Class 12 Chemistry Chapter 5, page 121).
Why A is wrong: A is wrong because it likely counts only the NH₃ molecules minus one, ignoring the coordinated Cl⁻.
Why C is wrong: C is wrong because it counts the 5 NH₃ but forgets the 1 Cl⁻ that is inside the coordination sphere — a common error of treating all chlorides as counter-ions (trap: counting counter-ions as all external).
Why D is wrong: D is wrong: 7 would need seven donor atoms. With 5 NH₃ and one coordinated Cl⁻ there are 6, and even counting all three Cl⁻ as ligands would give 8, not 7.
Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.
Given
A compound has the empirical formula PtCl₄·2NH₃. It is non-electrolyte (zero moles of AgCl with excess AgNO₃, no conductance in solution).
Required
Determine: (a) the coordination number of Pt, (b) the formula of the complex, (c) the primary and secondary valencies.
Concept
Non-electrolyte means zero ionizable ions — all species are inside the coordination sphere. Primary valency (oxidation state) is satisfied within the complex itself. Secondary valency equals coordination number (NCERT Class 12 Chemistry Chapter 5, page 121).
Formula / Rule
Coordination number = total number of ligand donor atoms bonded to the metal.
Primary valency of Pt = charge balanced by ligands within the sphere.
Substitution
Zero ionizable Cl⁻ → all 4 Cl⁻ are coordinated ligands.
2 NH₃ are also coordinated ligands.
All are monodentate.
Calculation
Coordination number = 4 (from Cl⁻) + 2 (from NH₃) = 6.
Complex formula: [PtCl₄(NH₃)₂] (no counter-ions outside the bracket).
Oxidation state of Pt: let x + 4(−1) + 2(0) = 0, so x = +4.
Primary valency = +4 (satisfied by the 4 Cl⁻ within the sphere).
Secondary valency = 6.
Note: The integer counts (4 Cl⁻, 2 NH₃) are exact counting numbers and do not limit significant figures.
Final answer
Coordination number of Pt⁴⁺ = 6. Formula: [PtCl₄(NH₃)₂]. Primary valency = 4, secondary valency = 6.
Common trap
Assuming that because there are 4 Cl⁻, some must be counter-ions — this leads to writing [PtCl₂(NH₃)₂]Cl₂ or similar. The conductance data (non-electrolyte) rules this out: zero ionizable ions means all chlorides are coordinated.
Similar NEET-style question
A compound CrCl₃·3NH₃ does not precipitate AgCl with AgNO₃ and is a non-electrolyte. Write the structural formula and find the coordination number of Cr.
*(Answer: [CrCl₃(NH₃)₃], coordination number = 6, primary valency = 3, secondary valency = 6.)*
---
Ligand: ion or neutral molecule donating electron pair to metal (Lewis base). Monodentate (1 site, e.g. NH₃, Cl⁻). Bidentate (2 sites, e.g. en, ox²⁻). Polydentate (chelate, e.g. EDTA⁴⁻ has 6 sites). Coordination number = number of donor atoms attached.
-- NCERT Class 12 Chemistry, Ch. 5, p. 121More in Coordination Compounds: 2 exam traps and mistakes · 2 formulas · 2 question patterns from its other lessons.
3 questions from NEET 2021, 2023, 2024. Answers verified against NTA official keys.
Homoleptic complex from the following complexes is
Ethylene diaminetetraacetate (EDTA) ion is :
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
counts counter ions as ligands
Treats all attached species as ligands
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
Test yourself on this topic with real past-paper questions:
Practice this topic →