CBSE Class 12 Chemistry Coordination Compounds MCQs Set 10

MCQs

Question. The oxidation of Ni in \( [Ni(CO)_4] \) is:
(a) 0
(b) 2
(c) 3
(d) 4
Answer: (a) 0
Explanation: CO is a neutral ligand and its oxidation state is zero. Since the overall charge on the complex is zero too, hence oxidation state of Ni is 0.

Question. Which of the following will give a white precipitate upon reacting with \( AgNO_3 \)?
(a) \( K_2[Pt(en)_2Cl_2] \)
(b) \( [Co(NH_3)_3Cl_3] \)
(c) \( [Cr(H_2O)_6]Cl_3 \)
(d) \( [Fe(H_2O)_3Cl_3] \)
Answer: (c) \( [Cr(H_2O)_6]Cl_3 \)
Explanation: \( AgNO_3 + [Cr(H_2O)_6]Cl_3 \rightarrow AgCl \) ppt Since Cl is outside the coordination sphere, it can react with \( AgNO_3 \) forming the white \( AgCl \) precipitate.

Question. The formula of the complex triamminetri(nitrito-O) cobalt (III) is:
(a) \( [Co(ONO)_3(NH_3)_3] \)
(b) \( [Co(NO_2)_3(NH_3)_3] \)
(c) \( [Co(ONO_2)_3(NH_3)_3] \)
(d) \( [Co(NO_2)(NH_3)_3] \)
Answer: (a) \( [Co(ONO)_3(NH_3)_3] \)
Explanation: According to IUPAC nomenclature, central metal atom with oxidation number +3, bonded with ligands \( :NH_3, ONO^- \) such that coordination formula is \( [Co(ONO)_3(NH_3)_3] \).

Question. How many ions are produced from the complex \( [Co(NH_3)_5Cl]Cl_2 \) in solution?
(a) 4
(b) 2
(c) 3
(d) 5
Answer: (c) 3
Explanation: \( [Co(NH_3)_5Cl]Cl_2 \rightarrow [Co(NH_3)_5Cl]^{2+}(aq) + 2Cl^- \)

Question. The coordination number of 'Co' in the complex \( [Co(en)_3]^{3+} \) is:
(a) 3
(b) 6
(c) 4
(d) 5
Answer: (b) 6
Explanation: Coordination number is the number of ligands joined to the central metal ion or atom. Since ethylenediamine is a bidentate ligand, Co has coordination number of 6.

Question. Which of the following is the most stable complex?
(a) \( [Fe(CO)_5] \)
(b) \( [Fe(H_2O)_6]^{3+} \)
(c) \( [Fe(C_2O_4)_3]^{3-} \)
(d) \( [Fe(CN)_6]^{3-} \)
Answer: (c) \( [Fe(C_2O_4)_3]^{3-} \)
Explanation: \( [Fe(C_2O_4)_3]^{3-} \) acts as the chelating complex ion because \( C_2O_4^{2-} \) is a bidentate ligand.

Question. Ambidentate ligands like \( NO_2^- \) and \( SCN^- \) are:
(a) unidentate
(b) didentate
(c) polydentate
(d) has variable denticity
Answer: (d) has variable denticity
Explanation: Ambidentate ligands like \( NO_2^- \) and \( SCN^- \) are such ligands which can attach through atoms of two different elements.

Question. The formula of the coordination compound Tetraammineaquachloridocobalt(III) chloride is:
(a) \( [Co(NH_3)_4(H_2O)Cl]Cl_2 \)
(b) \( [Co(NH_3)_4(H_2O)Cl]Cl_3 \)
(c) \( [Co(NH_3)_2(H_2O)Cl]Cl_2 \)
(d) \( [Co(NH_3)_4(H_2O)Cl]Cl \)
Answer: (a) \( [Co(NH_3)_4(H_2O)Cl]Cl_2 \)
Explanation: Tetraammineaquachloridocobalt (III) chloride is \( [Co(NH_3)_4(H_2O)Cl]Cl_2 \). Central atom – Cobalt(III). Coordination sphere ligands: Tetraammine - 4 \( NH_3 \) groups neutral ligand; Aqua – 1 \( H_2O \) group neutral ligand; Chlorido - 1 \( Cl \) group, negatively charged ligand, one negative charge. Counter ion - 2 Chloride ions. Since the valency of cobalt is 3+, one valency is satisfied with Cl in coordination sphere and 2 by chlorine counter ions. Hence, the formula of the coordination compound is \( [Co(NH_3)_4(H_2O)Cl]Cl_2 \).

Question. Which of the complex compounds give 2 moles of \( AgCl \) on reaction with \( AgNO_3 \)?
(a) 1 mole of \( CoCl_3 \cdot 6NH_3 \)
(b) 1 mole of \( CoCl_3 \cdot 5NH_3 \)
(c) 1 mole of \( CoCl_3 \cdot 4NH_3 \)
(d) All of the options
Answer: (b) 1 mole of \( CoCl_3 \cdot 5NH_3 \)
Explanation: In complex structure of 1 mole of \( CoCl_3 \cdot xNH_3 \), coordination number should be 6 in coordination sphere such that: \( [Co(NH_3)_6]Cl_3 \) gives 3 moles of AgCl. Similarly, \( [Co(NH_3)_5Cl]Cl_2 \) gives 2 moles of AgCl, \( [Co(NH_3)_4Cl_2]Cl \) gives 1 mole of AgCl.

Question. The crystal field splitting energy for octahedral (\( \Delta_o \)) and tetrahedral (\( \Delta_t \)) complexes is related as:
(a) \( \Delta_t = \frac{2}{9} \Delta_o \)
(b) \( \Delta_t = \frac{5}{9} \Delta_o \)
(c) \( \Delta_t = \frac{4}{9} \Delta_o \)
(d) \( \Delta_t = 2 \Delta_o \)
Answer: (c) \( \Delta_t = \frac{4}{9} \Delta_o \)
Explanation: In tetrahedral coordination entity formation, the d-orbital splitting is smaller than octahedral field splitting such that ratio of two energy levels (\( e_g \) and \( t_{2g} \)) for \( \Delta_o \) is 3:2 whereas, for \( \Delta_t \) is 2:3. Thus, \( \Delta_t = \frac{4}{9} \Delta_o \)

Question. Atomic number of Mn, Fe and Co are 25, 26, 27 respectively. Which of the following inner orbital octahedral complex ions are diamagnetic?
(a) \( [Co(NH_3)_6]^{3+} \)
(b) \( [Mn(CN)_6]^{3-} \)
(c) \( [Fe(CN)_6]^{3-} \)
(d) None of the options
Answer: (a) \( [Co(NH_3)_6]^{3+} \)
Explanation: Electronic configuration of \( Co^{3+} \) in \( [Co(NH_3)_6]^{3+} \) is \( [Ar] 3d^6 \). Strong field ligand \( NH_3 \) causes pairing, number of unpaired electron = 0. Magnetic nature = Diamagnetic.

Question. The colour of the coordination compounds depends on the crystal field splitting. What will be the correct order of absorption of wavelength of light in the visible region for the complexes, \( [Co(NH_3)_6]^{3+}, [Co(CN)_6]^{3-}, [Co(H_2O)_6]^{3+} \)?
(a) \( [Co(CN)_6]^{3-} > [Co(NH_3)_6]^{3+} > [Co(H_2O)_6]^{3+} \)
(b) \( [Co(NH_3)_6]^{3+} > [Co(H_2O)_6]^{3+} > [Co(CN)_6]^{3-} \)
(c) \( [Co(H_2O)_6]^{3+} > [Co(NH_3)_6]^{3+} > [Co(CN)_6]^{3-} \)
(d) \( [Co(CN)_6]^{3-} > [Co(NH_3)_6]^{3+} > [Co(H_2O)_6]^{3+} \)
Answer: (c) \( [Co(H_2O)_6]^{3+} > [Co(NH_3)_6]^{3+} > [Co(CN)_6]^{3-} \)
Explanation: \( \Delta_o \) values follow the order : \( [Co(H_2O)_6]^{3+} < [Co(NH_3)_6]^{3+} < [Co(CN)_6]^{3-} \) and \( \Delta_o \propto \frac{1}{\lambda} \). therefore, absorption wavelength follows the order : \( [Co(H_2O)_6]^{3+} > [Co(NH_3)_6]^{3+} > [Co(CN)_6]^{3-} \)

Question. Amongst the following, the most stable complex is:
(a) \( [Fe(H_2O)_6]^{3+} \)
(b) \( [Fe(NH_3)_6]^{3+} \)
(c) \( [Fe(C_2O_4)_3]^{3-} \)
(d) \( [FeCl_6]^{3-} \)
Answer: (c) \( [Fe(C_2O_4)_3]^{3-} \)
Explanation: We know that the stability of a complex increases by chelation. Therefore, the most stable complex is \( [Fe(C_2O_4)_3]^{3-} \).

Question. How many ions are produced from the complex \( Co(NH_3)_6Cl_2 \) in solution?
(a) 6
(b) 4
(c) 3
(d) 2
Answer: (c) 3
Explanation: The given complex can be written as \( [Co(NH_3)_6]Cl_2 \). Thus, \( [Co(NH_3)_6]^{2+} \) along with two \( Cl^- \) ions are produced.

Assertion and Reason Based MCQs

Directions: In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R). Mark the correct choice as:
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is NOT the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.

Question. Assertion (A): \( NF_3 \) is weaker ligand than \( N(CH_3)_3 \).
Reason (R): \( NF_3 \) ionizes to give \( F^- \) ions in aqueous solution.
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Answer: (b) Both (A) and (R) are true, but (R) is NOT the correct explanation of (A).
Explanation: \( F^- \) is highly electronegative atom which will withdraw electrons from nitrogen and create deficiency to donate electrons whereas \( -CH_3 \) is electron donating group that will increase electron density on nitrogen to donate. Thus, \( N(CH_3)_3 \) is strong field ligand than \( NF_3 \).

Question. Assertion (A): \( [Cr(H_2O)_6]Cl_2 \) and \( [Fe(H_2O)_6]Cl_2 \) are reducing in nature.
Reason (R): Due to presence of unpaired electrons, complexes are reducing in nature.
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Answer: (b) Both (A) and (R) are true, but (R) is NOT the correct explanation of (A).
Explanation: Due to formation of more stable complex ion after gaining electrons, these are reducing in nature. \( Cr^{+2} \) and \( Fe^{+2} \) will oxidise to \( Cr^{+3} \) and \( Fe^{+3} \) and achieve stable state.

Question. Assertion: Among \( [Co(NH_3)_6]^{3+} \) and \( [Co(en)_3]^{3+} \), coordination compound \( [Co(en)_3]^{3+} \) is a more stable complex.
Reason: \( en \) is a chelating ligand/bidentate ligand.
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Answer: (a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
Explanation: Since, \( en \) (ethylene diamine) is a chelating ligand/bidentate ligand, \( [Co(en)_3]^{3+} \) is a more stable complex as compared to the other one due to the formation of chelating ring.

Question. Assertion (A): \( [Fe(CN)_6]^{3-} \) ion shows magnetic moment corresponding to the two unpaired electrons.
Reason (R): \( [Fe(CN)_6]^{3-} \) it has \( d^2sp^3 \) type of hybridisation.
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Answer: (d) (A) is false, but (R) is true.
Explanation: \( [Fe(CN)_6]^{3-} \) ion shows magnetic moment corresponding to one unpaired electron.

Question. Assertion (A): \( [Cr(H_2O)_6]Cl_2 \) and \( [Fe(H_2O)_6]Cl_2 \) are reducing in nature.
Reason (R): Unpaired electrons are present in their d-orbitals.
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Answer: (b) Both (A) and (R) are true, but (R) is NOT the correct explanation of (A).
Explanation: In the complexes, Co exists as \( Co^{2+} \) and Fe as \( Fe^{2+} \). Both of the complexes become stable by oxidation of metal ion to \( Co^{3+} \) and \( Fe^{3+} \). It is due to the presence of weak field ligand, so paring of electrons does not take place and are easily removed because of excitation in \( e_g \) orbital.

Question. Assertion (A): Low spin tetrahedral complexes are rarely observed.
Reason (R): Crystal field splitting is less than pairing energy for tetrahedral complexes.
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Answer: (a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
Explanation: In tetrahedral complexes, the splitting of the d-orbitals is inverted and is smaller in comparison to octahedral complexes. The crystal field splitting energy is not large enough to force pairing and hence, low spin complexes are rarely observed.

Case-based MCQs

I. Read the passage given below and answer the following questions:
The crystal field theory (CFT) is an electrostatic model which considers the metal-ligand bond to be ionic arising purely from electrostatic interactions between the metal ion and the ligand. Ligands are treated as point charges in case of anions or dipoles in case of neutral molecules. The five d orbitals in an isolated gaseous metal atom/ion have same energy, i.e., they are degenerate. This degeneracy is maintained if a spherically symmetrical field of negative charges surrounds the metal atom/ion. However, when this negative field is due to ligands (either anions or the negative ends of dipolar molecules like \( NH_3 \) and \( H_2O \)) in a complex, it becomes asymmetrical and the degeneracy of the d orbitals is lifted. It results in splitting of the d orbitals.

Question. Consider the coordination compound, \( Na_2[Pt(CN)_4] \), identify the lewis acid.
(a) \( [Pt(CN)_4]^{2-} \)
(b) \( Na^+ \)
(c) \( Pt^{+2} \)
(d) \( CN^- \)
Answer: (c) \( Pt^{+2} \)
Explanation: Lewis acids are electron deficient and Pt has vacant orbital to accept electrons where as lewis base are electron donars i.e., \( CN^- \).

Question. The CFSE for octahedral \( [CoCl_6]^{4-} \) is 18,000 \( cm^{-1} \). The CFSE for tetrahedral \( [CoCl_4]^{2-} \) will be
(a) 18,000 \( cm^{-1} \)
(b) 16,000 \( cm^{-1} \)
(c) 8,000 \( cm^{-1} \)
(d) 20,000 \( cm^{-1} \)
Answer: (c) 8,000 \( cm^{-1} \)
Explanation: CFSE for tetrahedral complex is \( \Delta_t = \frac{4}{9} \Delta_o \).
\( \Delta_t = \frac{4}{9} \times 18,000 = 8,000 cm^{-1} \).

Question. An aqueous pink solution of cobalt(II) chloride changes to deep blue on addition of excess of HCl. This is because _____________.
(a) \( [Co(H_2O)_6]^{2+} \) is transformed into \( [CoCl_6]^{4-} \).
(b) \( [Co(H_2O)_6]^{2+} \) is transformed into \( [CoCl_4]^{2-} \).
(c) tetrahedral complexes have larger crystal field splitting than octahedral complex.
(d) None of the options
Answer: (b) \( [Co(H_2O)_6]^{2+} \) is transformed into \( [CoCl_4]^{2-} \).
Explanation: Due to d-d transition \( [Co(H_2O)_6]^{2+} \) where electron from \( t_{2g} \) shifts into \( e_g \), energy level absorbs light and shows complementary colour such that pink coloured aqueous \( CoCl_2 \) is obtained. Thus, on excess addition of HCl transforms \( [Co(H_2O)_6]^{2+} \) into \( [CoCl_4]^{2-} \).

Question. A chelating agent has two or more than two donor atoms to bind to a single metal ion. Which of the following is not a chelating agent?
(a) thiosulphato
(b) oxalato
(c) glycinato
(d) ethane-1,2-diamine
Answer: (a) thiosulphato


II. Read the passage given below and answer the following questions:
According to Valence Bond Theory, the metal atom or ion under the influence of ligands can use its (n − 1)d, ns, np or ns, np, nd orbitals for hybridisation to yield a set of equivalent orbitals of definite geometry such as octahedral, tetrahedral, square planar and so on. These hybridised orbitals are allowed to overlap with the ligand orbitals that can donate electron pairs for bonding. In these questions a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices:
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is NOT correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement
(d) Assertion is wrong statement but reason is correct statement.

Question. Assertion (A): In the diamagnetic octahedral complex, \( [Co(NH_3)_6]^{3+} \), the cobalt ion is in +3 oxidation state.
Reason (R): Six pairs of electrons, one from each \( NH_3 \) molecule, occupy the six hybrid orbitals.
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Answer: (b) Assertion and reason both are correct statements but reason is NOT correct explanation for assertion.

Question. Assertion (A): \( [NiCl_4]^{2-} \) is an inner orbital complex.
Reason (R): An inner orbital or low spin or spin paired complex uses inner d orbitals of the metal ion for hybridisation.
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Answer: (d) Assertion is wrong statement but reason is correct statement.

Question. Assertion (A): \( [Ni(CN)_4]^{2-} \) is square planar complexes, the hybridisation involved is \( dsp^2 \).
Reason (R): In \( [Ni(CN)_4]^{2-} \), nickel is in +2 oxidation state and has the electronic configuration \( 3d^8 \).
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Answer: (b) Assertion and reason both are correct statements but reason is NOT correct explanation for assertion.

Question. Assertion (A): The paramagnetic octahedral complex, \( [CoF_6]^{3-} \) uses outer orbital (4d) in hybridisation (\( sp^3d^2 \)).
Reason (R): It is a high spin complex.
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Answer: (b) Assertion and reason both are correct statements but reason is NOT correct explanation for assertion.