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 4). 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?
The coordination number of the central metal ion in [PtCl₆]²⁻ is:
In Werner's theory, the primary valency of the metal in [Co(NH₃)₆]Cl₃ is:
The coordination number of Co³⁺ in [Co(en)₂Cl₂]⁺ is:
In CoCl₃·4NH₃, the number of ionizable chloride ions (counter-ions outside the coordination sphere) is 1. What is the coordination number of Co³⁺?
EDTA⁴⁻ is a hexadentate ligand. What is the coordination number of Fe³⁺ in [Fe(EDTA)]⁻?
The compound CoCl₃·6NH₃ gives 3 moles of AgCl on treatment with excess AgNO₃. Which of the following correctly represents the complex?
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?
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 4).
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. 4Tetrahedral 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 |
Same spin-only formula but n depends on high-spin/low-spin from CFT.
| Symbol | Quantity | SI Unit |
|---|---|---|
| n | unpaired electrons | - |
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.
Coordination compound with given ligand asking for magnetic moment, color, or spin state.
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
Strong-field ligand (CN⁻, CO, NH₃ when applicable): Δ > P → low-spin. Weak-field (F⁻, Cl⁻, H₂O): Δ < P → high-spin. Use spectrochemical series.
11 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
Homoleptic complex from the following complexes is
Identify the incorrect statement from the following.
The IUPAC name of the complex- [Ag(H O) ][Ag(CN) ] is: 2 2 2
Ethylene diaminetetraacetate (EDTA) ion is :
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
forgets weak vs strong ligand
Default low-spin always
wrong order ligands
Uses non-alphabetical order
counts counter ions as ligands
Treats all attached species as ligands
Test yourself on this topic with real past-paper questions:
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