JEE Chemistry Coordination Compounds MCQs Set 04

Multiple Choice Questions (MCQs) for JEE Chemistry: Coordination Compounds

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Multiple Choice Questions

Question 1: FeCl3·4H2O is actually
(a) [Fe(H2O)4]Cl3
(b) [Fe(H2O)3Cl]Cl2·H2O
(c) [Fe(H2O)4Cl2]Cl
(d) [Fe(H2O)2Cl2]Cl·H2O
Show Answer & Explanation

Answer: (c) [Fe(H2O)4Cl2]Cl

Explanation:
1. Fe(III) is octahedral, so its coordination number is 6
2. Only [Fe(H2O)4Cl2]Cl has four water molecules and two chloride ligands (CN = 6) with the formula FeCl3·4H2O.

Teacher's Note:
1. The other options have a coordination number of 4 or the wrong amount of water.
2. Check both the composition and the coordination number.

Question 2: The number of isomeric forms in which [Co(NH3)4Cl2]+ can occur is
(a) 2
(b) 3
(c) 4
(d) 1
Show Answer & Explanation

Answer: (a) 2

Explanation:
1. M a4 b2 octahedral: cis and trans only.

Teacher's Note:
1. Both forms are optically inactive.
2. The trans form is violet and the cis form is green.

Question 3: The IUPAC name of Fe(CO)5 is
(a) pentacarbonylferrate(0)
(b) pentacarbonylferrate(III)
(c) pentacarbonyliron(0)
(d) pentacarbonyliron(II)
Show Answer & Explanation

Answer: (c) pentacarbonyliron(0)

Explanation:
1. The complex is neutral, so the metal name is plain iron
2. CO is neutral, so the oxidation number of Fe is 0.

Teacher's Note:
1. Anionic complexes end in –ate, neutral and cationic keep the metal name.
2. The oxidation state is written in Roman numerals (0 as zero).

Question 4: The complex which absorbs light of the longest wavelength (lowest energy band) is
(a) [NiCl4]2−
(b) [Ni(CO)4]
(c) [Ni(CN)4]2−
(d) [Ni(H2O)6]2+
Show Answer & Explanation

Answer: (a) [NiCl4]2−

Explanation:
1. Longest wavelength means smallest energy gap Δ
2. Tetrahedral complexes with a weak ligand Cl− have the smallest splitting, so [NiCl4]2− absorbs at the longest wavelength.

Teacher's Note:
1. E = hc/λ: small Δ means large λ.
2. Δt is about 4/9 of Δo.

Question 5: K4[Fe(CN)6] reacts with FeCl3 to form
(a) K3[Fe(CN)6]
(b) K4[Fe(CN)3Cl3]
(c) K3[Fe(CN)5Cl]
(d) KFe[Fe(CN)6]
Show Answer & Explanation

Answer: (d) KFe[Fe(CN)6]

Explanation:
1. Ferrocyanide with Fe3+ gives Prussian blue, the soluble form being KFe[Fe(CN)6].

Teacher's Note:
1. The insoluble form is Fe4[Fe(CN)6]3.
2. This is a test for Fe3+.

Question 6: The stability constants of the complexes formed by a metal ion M2+ with NH3, CN− and H2O are of the order of 1015, 1027 and 1011 respectively. The ligand that forms the most stable complex is
(a) NH3
(b) CN−
(c) H2O
(d) all form equally stable complexes
Show Answer & Explanation

Answer: (b) CN−

Explanation:
1. A larger stability constant means a more stable complex
2. 1027 is the largest, so CN− forms the most stable complex with M2+.

Teacher's Note:
1. Stability constant K = [MLn]/([M][L]n).
2. The reaction is a Lewis acid–base equilibrium.

Question 7: Match the complexes (A) [Ni(CN)4]2−, (B) [Pt(NH3)6]4+, (C) [Ag(NH3)2]+, (D) [Zn(NH3)4]2+ with the geometries (a) octahedral, (b) square planar, (c) tetrahedral, (d) linear. The correct pairing is
(a) A–b, B–a, C–d, D–c
(b) A–a, B–b, C–c, D–d
(c) A–c, B–d, C–b, D–a
(d) A–b, B–c, C–d, D–a
Show Answer & Explanation

Answer: (a) A–b, B–a, C–d, D–c

Explanation:
1. [Ni(CN)4]2−: dsp2, square planar; [Pt(NH3)6]4+: octahedral
2. [Ag(NH3)2]+: sp, linear; [Zn(NH3)4]2+: sp3, tetrahedral.

Teacher's Note:
1. Coordination number 2 is linear, 6 is octahedral.
2. For CN = 4 decide between tetrahedral and square planar from the metal and ligand.

Question 8: The effective atomic number of Cr in Cr(CO)6 is
(a) 36
(b) 38
(c) 28
(d) 54
Show Answer & Explanation

Answer: (a) 36

Explanation:
1. EAN = 24 − 0 + 2 × 6 = 36
2. This equals the atomic number of krypton.

Teacher's Note:
1. EAN = Z − oxidation state + 2 × coordination number.
2. Metal carbonyls follow the EAN rule closely.

Question 9: For a 3d metal ion forming a complex with EDTA, the ratio of moles of metal to EDTA in the complex is
(a) 1 : 6
(b) 6 : 1
(c) 1 : 1
(d) a 3d metal ion cannot bond with EDTA
Show Answer & Explanation

Answer: (c) 1 : 1

Explanation:
1. EDTA is hexadentate, so one EDTA wraps around one metal ion and fills all six octahedral positions
2. The ratio is 1 : 1.

Teacher's Note:
1. Denticity 6 and coordination number 6 mean one ligand per metal ion.
2. EDTA is used to estimate hardness (Ca2+, Mg2+) for this reason.

Question 10: The formula of tris(ethylenediamine)cobalt(III) sulphate is
(a) [Co(en)2SO4]
(b) [Co(en)3SO4]
(c) [Co(en)3]SO4
(d) [Co(en)3]2(SO4)3
Show Answer & Explanation

Answer: (d) [Co(en)3]2(SO4)3

Explanation:
1. The cation is [Co(en)3]3+ and sulphate is 2−
2. Balancing charge: two cations and three sulphates, [Co(en)3]2(SO4)3.

Teacher's Note:
1. Cross-multiply charges to find the ratio.
2. Tris is used because the ligand name contains a number (ethylene diamine).

Question 11: Among [Ni(CO)4], [Ni(CN)4]2− and [NiCl4]2−, which is correct?
(a) [Ni(CO)4] and [NiCl4]2− are diamagnetic and [Ni(CN)4]2− is paramagnetic
(b) [NiCl4]2− and [Ni(CN)4]2− are diamagnetic and [Ni(CO)4] is paramagnetic
(c) [Ni(CO)4] and [Ni(CN)4]2− are diamagnetic and [NiCl4]2− is paramagnetic
(d) [Ni(CO)4] is diamagnetic and [NiCl4]2− and [Ni(CN)4]2− are paramagnetic
Show Answer & Explanation

Answer: (c) [Ni(CO)4] and [Ni(CN)4]2− are diamagnetic and [NiCl4]2− is paramagnetic

Explanation:
1. Ni(CO)4: Ni(0) d10, sp3, diamagnetic
2. [Ni(CN)4]2−: d8 with strong field, dsp2, diamagnetic
3. [NiCl4]2−: d8 with weak field, sp3, two unpaired electrons, paramagnetic.

Teacher's Note:
1. Ni(II) is square planar with strong ligands and tetrahedral with weak ligands.
2. Tetrahedral d8 has two unpaired electrons.

Question 12: The compound Na2[Fe(CN)5NO] is called
(a) sodium pentacyanonitrosonium ferrate(II)
(b) sodium nitroprusside
(c) sodium nitrosoferrocyanide
(d) 1 and 2 both
Show Answer & Explanation

Answer: (d) 1 and 2 both

Explanation:
1. The anion is [Fe(CN)5(NO)]2−: with NO+ (nitrosonium) the iron is +2
2. Systematic name: sodium pentacyanonitrosoniumferrate(II); its common name is sodium nitroprusside.

Teacher's Note:
1. Nitroprusside is used as a test for sulphide (purple colour).
2. NO+ is a three-electron-count ligand here.

Question 13: The neutral ligand among the following is
(a) chlorido
(b) hydroxido
(c) ammine
(d) oxalato
Show Answer & Explanation

Answer: (c) ammine

Explanation:
1. Chlorido (Cl−), hydroxido (OH−) and oxalato (C2O42−) are anionic
2. Ammine (NH3) is neutral.

Teacher's Note:
1. NH3 is named ammine (two m's) while amines are organic.
2. Other neutral ligands: H2O (aqua), CO (carbonyl), NO (nitrosyl).

Question 14: Which of the following complexes does not have chelation?
(a) K3[Al(C2O4)3]
(b) [Pt(en)2]Cl2
(c) [Ag(NH3)2]Cl
(d) K2[Ni(EDTA)]
Show Answer & Explanation

Answer: (c) [Ag(NH3)2]Cl

Explanation:
1. Oxalate, en and EDTA are bidentate or polydentate and form rings
2. NH3 is monodentate, so [Ag(NH3)2]Cl has no chelate ring.

Teacher's Note:
1. Chelation needs a ligand with two or more donor sites.
2. Chelates are more stable than analogues of monodentate ligands.

Question 15: Which of the following is an organometallic compound?
(a) Ti(OCOCH3)4
(b) Ti(OC6H5)4
(c) Ti(OC2H5)3C2H5
(d) Ti(OC2H5)4
Show Answer & Explanation

Answer: (c) Ti(OC2H5)3C2H5

Explanation:
1. Only Ti(OC2H5)3C2H5 has a direct Ti–C bond; the others have Ti–O–C bonds.

Teacher's Note:
1. Count bonds from the metal to carbon, not to oxygen.
2. Acetates and alkoxides are not organometallic.

Question 16: A blue colour is not obtained when
(a) NH4OH is added to CuSO4
(b) CuSO4 solution reacts with K4[Fe(CN)6]
(c) FeCl3 reacts with K4[Fe(CN)6]
(d) anhydrous white CuSO4 is dissolved in water
Show Answer & Explanation

Answer: (b) CuSO4 solution reacts with K4[Fe(CN)6]

Explanation:
1. (1) gives deep blue [Cu(NH3)4]2+; (3) gives Prussian blue; (4) gives blue [Cu(H2O)4]2+
2. CuSO4 with K4[Fe(CN)6] gives a chocolate-brown precipitate of Cu2[Fe(CN)6].

Teacher's Note:
1. Chocolate-brown Cu2[Fe(CN)6] is a test for Cu2+.
2. The brown colour is not blue.

Question 17: Which of [Fe(CN)6]3−, [Co(NO2)6]3− and [Mn(CN)6]3− are low spin complexes?
(a) [Fe(CN)6]3− only
(b) [Co(NO2)6]3− only
(c) [Mn(CN)6]3− only
(d) all of them
Show Answer & Explanation

Answer: (d) all of them

Explanation:
1. CN− and NO2− are strong field ligands
2. With a strong field ligand, pairing occurs in t2g: all three are low spin.

Teacher's Note:
1. Spectrochemical series: I− < Br− < Cl− < F− < OH− < H2O < NH3 < en < NO2− < CN− < CO.
2. Strong field ligands give low spin.

Question 18: Which of the stability orders is incorrect?
(a) [Fe(C2O4)3]3− > [Fe(H2O)6]3+
(b) [Fe(EDTA)]2− > [Fe(en)3]3+
(c) [Ni(en)2]2+ > [Ni(dmg)2]
(d) [Fe(CN)6]3− > [Fe(CN)6]4−
Show Answer & Explanation

Answer: (c) [Ni(en)2]2+ > [Ni(dmg)2]

Explanation:
1. Chelates are more stable than aqua complexes, and EDTA is more stable than en
2. [Ni(dmg)2] is extremely stable because of two chelate rings and hydrogen bonding, more than [Ni(en)2]2+, so order 3 is wrong.

Teacher's Note:
1. Higher charge on the metal also raises stability.
2. Chelate effect: more rings, more stability.

Question 19: For the complex Fe(CO)x, which statement is wrong?
(a) it is a σ–π bonded organometallic compound
(b) the value of x is 6
(c) CO is a π-acid ligand in the complex
(d) it has a trigonal bipyramidal shape
Show Answer & Explanation

Answer: (b) the value of x is 6

Explanation:
1. EAN rule: 26 + 2x = 36, so x = 5
2. Fe(CO)5 is trigonal bipyramidal (dsp3), so the statement x = 6 is wrong.

Teacher's Note:
1. CO donates a σ pair and accepts back π electrons.
2. Cr(CO)6 and Ni(CO)4 follow the same rule.

Question 20: The magnetic moment of a t2g6 eg2 system is
(a) 2.83 BM
(b) 1.73 BM
(c) 3.87 BM
(d) 4.92 BM
Show Answer & Explanation

Answer: (a) 2.83 BM

Explanation:
1. t2g6 eg2 is d8 with two unpaired electrons
2. μ = √[2(2 + 2)] = √8 = 2.83 BM.

Teacher's Note:
1. d8 octahedral always has two unpaired electrons.
2. Remember μ = √[n(n + 2)] BM.

Question 21: The effective atomic number of iron in K4[Fe(CN)6] is
(a) 35
(b) 34
(c) 36
(d) 38
Show Answer & Explanation

Answer: (c) 36

Explanation:
1. Fe is +2 in the complex: EAN = 26 − 2 + 2 × 6 = 36.

Teacher's Note:
1. Fe2+ has 24 electrons and gains 12 from six CN−.
2. 36 is the atomic number of krypton.

Question 22: [Co(NO2)(NH3)5]Cl2 and [Co(ONO)(NH3)5]Cl2 are examples of
(a) coordination isomerism
(b) ionisation isomerism
(c) geometrical isomerism
(d) linkage isomerism
Show Answer & Explanation

Answer: (d) linkage isomerism

Explanation:
1. The ligand NO2− binds through N (nitro) in one and through O (nitrito) in the other
2. This is linkage isomerism.

Teacher's Note:
1. Linkage isomerism needs an ambidentate ligand.
2. SCN− and CN− give the same behaviour.

Question 23: Which of the following orders is/are correct? (i) Δo: [NiCl6]4− > [NiCl4]2−; (ii) CFSE: [Cr(en)3]Cl3 > [Cr(H2O)6]Cl3; (iii) reducing behaviour: [Co(CN)6]4− > [Co(CN)6]3−
(a) (i) only
(b) (i) and (ii) only
(c) (ii) and (iii) only
(d) (i), (ii) and (iii)
Show Answer & Explanation

Answer: (d) (i), (ii) and (iii)

Explanation:
1. (i) Δt = (4/9)Δo, so octahedral splitting is larger
2. (ii) en is a stronger ligand than H2O, so Δo and the CFSE of the d3 ion are larger
3. (iii) Co(II) easily loses an electron to give Co(III): [Co(CN)6]4− is a strong reducing agent.

Teacher's Note:
1. A stronger field gives a larger CFSE for the same ion.
2. [Co(CN)6]4− has an electron in an eg orbital.

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