Coordination Number and Nomenclature of Coordination Compounds Introduction to coordination compounds. Werner's theory; ligands, coordination number, denticity. chelation; IUPAC nomenclature of mononuclear co-ordination compounds,: NEET MCQs with Answers

15 free questions on Coordination Number and Nomenclature of Coordination Compounds Introduction to coordination compounds. Werner's theory; ligands, coordination number, denticity. chelation; IUPAC nomenclature of mononuclear co-ordination compounds, for NEET (UG), starting with previous-year NEET and AIPMT questions. Try each one, then tap Show answer for the answer and a short explanation.

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Question 1

Previous-year question
The anion of acetylacetone (acac) forms the chelate Co(acac)₃ with Co³⁺. The rings of the chelate are [NEET 2013]
  1. five membered
  2. four membered
  3. six membered
  4. three membered
Show answer
Answer: (C) six membered
Explanation: Acetylacetone, CH₃COCH₂COCH₃, loses the acidic proton from the central CH₂ group to give the acetylacetonate anion. The negative charge is delocalised over the O—C—C—C—O framework, and the ligand binds the metal through both oxygen atoms. Counting the atoms in the closed ring gives: Co, O, C, C, C and back to O, which is six atoms in all, so each chelate ring is six-membered. Six-membered rings of this kind, stabilised further by delocalisation, are particularly stable, and Co(acac)₃ contains three of them.

Question 2

Previous-year question
The correct order of the stoichiometries of AgCl formed when excess AgNO₃ is treated with CoCl₃·6NH₃, CoCl₃·5NH₃ and CoCl₃·4NH₃ respectively is [NEET 2017]
  1. 3AgCl, 1AgCl, 2AgCl
  2. 3AgCl, 2AgCl, 1AgCl
  3. 2AgCl, 3AgCl, 1AgCl
  4. 1AgCl, 3AgCl, 2AgCl
Show answer
Answer: (B) 3AgCl, 2AgCl, 1AgCl
Explanation: The formulations are [Co(NH₃)₆]Cl₃ (three ionisable Cl⁻), [Co(NH₃)₅Cl]Cl₂ (two ionisable) and [Co(NH₃)₄Cl₂]Cl (one ionisable), so the amounts of AgCl are 3, 2 and 1 mole respectively.

Question 3

Previous-year question
The name of the complex ion [Fe(CN)₆]³⁻ is [AIPMT 2015]
  1. hexacyanoiron(III) ion
  2. hexacyanitoferrate(III) ion
  3. tricyanoferrate(III) ion
  4. hexacyanidoferrate(III) ion
Show answer
Answer: (D) hexacyanidoferrate(III) ion
Explanation: The complex is an anion, so the metal takes the Latin stem with the suffix -ate, i.e. ferrate. Six cyanido ligands give the prefix hexa-, and iron is in the +3 state.

Question 4

Previous-year question
The sum of the coordination number and the oxidation number of the metal M in the complex [M(en)₂(C₂O₄)]Cl (en = ethylenediamine) is [AIPMT 2015]
  1. 9
  2. 6
  3. 7
  4. 8
Show answer
Answer: (A) 9
Explanation: Two bidentate en plus one bidentate oxalate give a coordination number of 6. For the charge, x + 0 + (–2) = +1, so the oxidation number is +3. The required sum is 6 + 3 = 9.

Question 5

Previous-year question
Cobalt(III) chloride forms several octahedral complexes with ammonia. Which of the following will NOT give a test for chloride ions with AgNO₃ at 25 °C? [AIPMT 2015]
  1. CoCl₃·4NH₃
  2. CoCl₃·5NH₃
  3. CoCl₃·6NH₃
  4. CoCl₃·3NH₃
Show answer
Answer: (D) CoCl₃·3NH₃
Explanation: With only three ammonia molecules all three chlorides must enter the coordination sphere to complete the octahedron, giving the neutral non-electrolyte [Co(NH₃)₃Cl₃]. Since no chloride is ionisable, no precipitate forms with AgNO₃.

Question 6

Previous-year question
Which of the following ligands is expected to be bidentate? [AIPMT 1994]
  1. CH₃NH₂
  2. CH₃C≡N
  3. Br⁻
  4. C₂O₄²⁻
Show answer
Answer: (D) C₂O₄²⁻
Explanation: A bidentate ligand must have two donor atoms placed so that both can reach the metal at once and close a stable ring. The oxalate ion, C₂O₄²⁻, has two carboxylate groups joined by a C—C bond. One oxygen from each group coordinates to the metal, closing a five-membered chelate ring — so it is bidentate. ✔ Methylamine, CH₃NH₂, donates only through its single nitrogen — monodentate. Acetonitrile, CH₃C≡N, donates only through the nitrogen lone pair — monodentate. The bromide ion has just one atom available — monodentate.

Question 7

Previous-year question
Which of the following will exhibit maximum ionic conductivity? [AIPMT 2001]
  1. K₄[Fe(CN)₆]
  2. [Co(NH₃)₆]Cl₃
  3. [Cu(NH₃)₄]Cl₂
  4. [Ni(CO)₄]
Show answer
Answer: (A) K₄[Fe(CN)₆]
Explanation: Ionic conductivity in solution depends on the number of ions produced per formula unit. K₄[Fe(CN)₆] → 4K⁺ + [Fe(CN)₆]⁴⁻ = 5 ions ✔ [Co(NH₃)₆]Cl₃ → [Co(NH₃)₆]³⁺ + 3Cl⁻ = 4 ions [Cu(NH₃)₄]Cl₂ → [Cu(NH₃)₄]²⁺ + 2Cl⁻ = 3 ions [Ni(CO)₄] is a neutral, non-ionising molecule = 0 ions With five ions per formula unit, potassium hexacyanidoferrate(II) gives the highest conductivity.
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Question 8

Previous-year question
An example of a double salt is [AIPMT 1989]
  1. bleaching powder
  2. K₄[Fe(CN)₆]
  3. hypo
  4. potash alum
Show answer
Answer: (D) potash alum
Explanation: Potash alum, K₂SO₄·Al₂(SO₄)₃·24H₂O, is a double salt: it crystallises from two simple salts and in water gives all the ions K⁺, Al³⁺ and SO₄²⁻, each detectable by its usual test. K₄[Fe(CN)₆] is a coordination compound that keeps the [Fe(CN)₆]⁴⁻ ion intact, bleaching powder is a mixed salt and hypo, Na₂S₂O₃·5H₂O, is a simple salt.

Question 9

Previous-year question
The coordination number and oxidation state of Cr in K₃[Cr(C₂O₄)₃] are respectively [AIPMT 1995]
  1. 3 and +3
  2. 3 and 0
  3. 6 and +3
  4. 4 and +2
Show answer
Answer: (C) 6 and +3
Explanation: Coordination number: oxalate is bidentate, so three oxalate ligands supply 3 × 2 = 6 donor oxygen atoms. The coordination number is 6 and the complex is octahedral. Oxidation state: the three potassium ions give the complex anion a charge of −3, and each oxalate is −2, so 3(+1) + x + 3(−2) = 0 ⟹ 3 + x − 6 = 0 ⟹ x = +3 Hence coordination number 6 and oxidation state +3.

Question 10

Previous-year question
The formula of dichloridobis(urea)copper(II) is [AIPMT 1997]
  1. [Cu{O=C(NH₂)₂Cl}]Cl
  2. [CuCl₂{O=C(NH₂)₂}₂]
  3. [Cu{O=C(NH₂)₂}₂]Cl₂
  4. [CuCl₂{O=C(NH₂)₂H₂}]
Show answer
Answer: (B) [CuCl₂{O=C(NH₂)₂}₂]
Explanation: The name has no separate counter ion, so both chlorido ligands and both urea molecules lie inside the coordination sphere and the complex is neutral. Urea is a neutral monodentate ligand binding through oxygen, and (+2) + 2(−1) + 2(0) = 0 confirms the formula [CuCl₂{O=C(NH₂)₂}₂].

Question 11

Previous-year question
A coordination compound of cobalt has a molecular formula containing five ammonia molecules, one nitro group and two chlorine atoms per cobalt atom. One mole of this compound produces three moles of ions in aqueous solution, and on treatment with excess AgNO₃ it gives two moles of AgCl precipitate. The ionic formula of this complex is [AIPMT 1998]
  1. [Co(NH₃)₅(NO₂)]Cl₂
  2. [Co(NH₃)₅Cl][Cl(NO₂)]
  3. [Co(NH₃)₄(NO₂)Cl][(NH₃)Cl]
  4. [Co(NH₃)₅][(NO₂)₂Cl₂]
Show answer
Answer: (A) [Co(NH₃)₅(NO₂)]Cl₂
Explanation: Two moles of AgCl means both chlorine atoms are ionisable and must lie outside the coordination sphere. The nitro group and all five ammonia molecules are therefore inside, giving the complex ion [Co(NH₃)₅(NO₂)]²⁺ with a coordination number of six. The formula must then be [Co(NH₃)₅(NO₂)]Cl₂, which ionises as: [Co(NH₃)₅(NO₂)]Cl₂ → [Co(NH₃)₅(NO₂)]²⁺ + 2Cl⁻ That is one complex cation plus two chloride ions — three moles of ions in total, matching the data exactly.

Question 12

Previous-year question
The IUPAC name of [Pt(NH₃)₃(Br)(NO₂)Cl]Cl is [AIPMT 1998]
  1. Triamminebromidochloridonitroplatinum(IV) chloride
  2. Triamminebromidonitrochloridoplatinum(IV) chloride
  3. Triamminechloridobromidonitroplatinum(IV) chloride
  4. Triamminenitrochloridobromidoplatinum(IV) chloride
Show answer
Answer: (A) Triamminebromidochloridonitroplatinum(IV) chloride
Explanation: Ligands are cited in alphabetical order ignoring multiplying prefixes: ammine, bromido, chlorido, nitro. The oxidation state follows from x + 3(0) + (−1) + (−1) + (−1) = +1, so x = +4, and the outer chloride is named last as the counter ion.

Question 13

Previous-year question
The coordination number of Ni in [Ni(C₂O₄)₃]⁴⁻ is [AIPMT 2001]
  1. 3
  2. 6
  3. 4
  4. 2
Show answer
Answer: (B) 6
Explanation: Coordination number is the number of donor atoms directly bonded to the central metal, not the number of ligand molecules. The oxalate ion, C₂O₄²⁻, is a bidentate ligand: it attaches through two of its oxygen atoms and closes a five-membered chelate ring. With three oxalate ligands the count is 3 × 2 = 6, so nickel has a coordination number of six and the complex is octahedral. Choosing 3 would be the mistake of counting ligands rather than donor atoms.

Question 14

Previous-year question
CuSO₄ reacts with KCN to form CuCN, which is insoluble in water. It dissolves in excess KCN due to the formation of the complex [AIPMT 2002]
  1. K₂[Cu(CN)₄]
  2. K₃[Cu(CN)₄]
  3. Cu(CN)₂
  4. Cu[KCu(CN)₄]
Show answer
Answer: (B) K₃[Cu(CN)₄]
Explanation: The reactions are: CuSO₄ + 2KCN → K₂SO₄ + Cu(CN)₂ (unstable) 2Cu(CN)₂ → 2CuCN↓ + (CN)₂↑ (insoluble cuprous cyanide and cyanogen) CuCN + 3KCN → K₃[Cu(CN)₄] (soluble) Cyanide reduces copper(II) to copper(I), so the metal in the final complex is Cu⁺. The charge on the complex anion is therefore (+1) + 4(−1) = −3, which requires three potassium ions — potassium tetracyanidocuprate(I).

Question 15

Previous-year question
The hypothetical complex chlorodiaquatriamminecobalt(III) chloride can be represented as [AIPMT 2002 · NCERT]
  1. [CoCl(NH₃)₃(H₂O)₂]Cl₂
  2. [Co(NH₃)₃(H₂O)Cl₃]
  3. [Co(NH₃)₃(H₂O)₂Cl]
  4. [Co(NH₃)₃(H₂O)₃]Cl₃
Show answer
Answer: (A) [CoCl(NH₃)₃(H₂O)₂]Cl₂
Explanation: Reading the name backwards: the ligands inside the sphere are one chlorido, two aqua and three ammine, and the metal is cobalt(III). That fixes the complex ion as [CoCl(NH₃)₃(H₂O)₂]ⁿ⁺ with a coordination number of 1 + 2 + 3 = 6, as expected for Co(III). Charge on the ion: (+3) + (−1) + 2(0) + 3(0) = +2. The final word 'chloride' supplies the counter ions, and two are needed to neutralise the +2 cation. Hence the formula is [CoCl(NH₃)₃(H₂O)₂]Cl₂.

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