Level 1 — RecognitionCoordination Chemistry

Coordination Chemistry

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Time: 20 minutes | Total Marks: 30

Instructions: Answer all questions. For True/False items, a correct justification is required for full marks.

Section A — Multiple Choice (1 mark each) [10 marks]

Q1. In Werner's theory, the term used for the number of secondary valences is: (a) oxidation number (b) coordination number (c) primary valence (d) ligancy of anions

Q2. Which of the following is an ambidentate ligand? (a) $NH_3$ (b) $en$ (c) $SCN^-$ (d) $Cl^-$

Q3. The IUPAC name of $[Co(NH_3)_6]Cl_3$ is: (a) hexaamminecobalt(III) chloride (b) hexaamminecobalt(II) trichloride (c) cobalthexammine chloride (d) hexaaminecobalt(III) chloride

Q4. A coordination number of 4 with a $d^8$ metal ion and strong field ligands most commonly gives which geometry? (a) tetrahedral (b) square planar (c) octahedral (d) linear

Q5. $[Co(NH_3)_5Cl]SO_4$ and $[Co(NH_3)_5SO_4]Cl$ exhibit which isomerism? (a) linkage (b) hydrate (c) ionization (d) coordination

Q6. The EAN of the central metal in $[Fe(CN)_6]^{4-}$ (Fe: Z = 26) is: (a) 34 (b) 35 (c) 36 (d) 54

Q7. In VBT, the hybridization for an inner-orbital octahedral complex is: (a) $sp^3$ (b) $sp^3d^2$ (c) $d^2sp^3$ (d) $dsp^2$

Q8. For an octahedral complex, the crystal field splitting produces: (a) $e_g$ (lower) and $t_{2g}$ (higher) (b) $t_{2g}$ (lower) and $e_g$ (higher) (c) two degenerate sets equal in energy (d) three sets of orbitals

Q9. Jahn–Teller distortion is most strongly expected for: (a) $d^3$ octahedral (b) $d^5$ high-spin octahedral (c) $d^9$ octahedral (d) $d^{10}$ octahedral

Q10. The chelate effect refers to the observation that chelate complexes are: (a) less stable than comparable monodentate complexes (b) more stable than comparable monodentate complexes (c) always coloured (d) always diamagnetic

Section B — Matching (1 mark each) [8 marks]

Q11. Match the complex/ion in Column I with its property in Column II.

Column I	Column II
(i) $[Ni(CN)_4]^{2-}$	(P) linear, $sp$
(ii) $[Ag(NH_3)_2]^+$	(Q) octahedral, $d^2sp^3$
(iii) $[CoF_6]^{3-}$	(R) square planar, $dsp^2$
(iv) $[Co(NH_3)_6]^{3+}$	(S) octahedral, $sp^3d^2$

Q12. Match the species/molecule in Column I with its biological/medicinal role in Column II.

Column I	Column II
(i) Haemoglobin	(P) Mg centre, photosynthesis
(ii) Chlorophyll	(Q) Co centre, vitamin
(iii) Vitamin $B_{12}$	(R) anticancer drug
(iv) Cisplatin	(S) Fe centre, $O_2$ transport

Section C — True/False with Justification (2 marks each) [12 marks]

1 mark for correct T/F, 1 mark for correct justification.

Q13. Water lies higher than cyanide in the spectrochemical series. (T/F + justify)

Q14. $[Ni(CO)_4]$ is tetrahedral and diamagnetic. (T/F + justify)

Q15. cis- and trans- $[Pt(NH_3)_2Cl_2]$ are geometrical isomers, and only the cis form is optically active. (T/F + justify)

Q16. For a free ion, the spin-only magnetic moment of a $d^5$ high-spin complex is about $5.92\ \mu_B$ . (T/F + justify)

Q17. $\Delta_{tet}$ is approximately equal to $\tfrac{4}{9}\,\Delta_{oct}$ , so tetrahedral complexes are usually low-spin. (T/F + justify)

Q18. In $[Co(NH_3)_6]^{3+}$ , the observed colour arises purely from a fully spin- and Laporte-allowed d–d transition. (T/F + justify)

Answer keyMark scheme & solutions

Section A

Q1. (b) coordination number. Werner called primary valences the ionizable (oxidation) charges and secondary valences the fixed coordination number. (1)

Q2. (c) $SCN^-$ . It can bind through N (thiocyanato-N) or S (thiocyanato-S); $NH_3, Cl^-$ are monodentate single-donor, en is bidentate. (1)

Q3. (a) hexaamminecobalt(III) chloride. Ammine has double "m"; oxidation state III (since 3 Cl⁻ counter ions). (1)

Q4. (b) square planar. Strong-field $d^8$ (e.g. Ni²⁺, Pt²⁺) favours $dsp^2$ square planar geometry. (1)

Q5. (c) ionization. The $Cl^-$ and $SO_4^{2-}$ swap between inside/outside coordination sphere, giving different ions in solution. (1)

Q6. (c) 36. EAN $= 26 - 2 + (6\times2) = 24 + 12 = 36$ . (1)

Q7. (c) $d^2sp^3$ . Inner-orbital uses inner (n−1)d orbitals → $d^2sp^3$ ; outer uses $sp^3d^2$ . (1)

Q8. (b) $t_{2g}$ lower, $e_g$ higher. In octahedral field the $t_{2g}$ set is stabilized (−0.4Δₒ) and $e_g$ destabilized (+0.6Δₒ). (1)

Q9. (c) $d^9$ . Unequal occupation of $e_g$ orbitals ( $t_{2g}^6 e_g^3$ ) gives strong Jahn–Teller distortion. (1)

Q10. (b) more stable. Chelation increases stability due to a favourable entropy contribution (more free particles released). (1)

Section B

Q11. (i)→R, (ii)→P, (iii)→S, (iv)→Q (1 each = 4) Ni²⁺ $d^8$ with CN⁻ (strong) → square planar $dsp^2$ ; Ag⁺ CN=2 → linear $sp$ ; $CoF_6^{3-}$ F⁻ weak → outer $sp^3d^2$ ; $[Co(NH_3)_6]^{3+}$ strong → inner $d^2sp^3$ .

Q12. (i)→S, (ii)→P, (iii)→Q, (iv)→R (1 each = 4)

Section C

Q13. False. Spectrochemical order: ...H₂O < NH₃ < ... < CN⁻. CN⁻ is a much stronger field ligand and lies far above H₂O. (T/F 1 + justify 1)

Q14. True. Ni is $3d^8 4s^2$ ; CO is strong field causing pairing, Ni becomes 3d¹⁰ (0 in valence), sp³ hybridization → tetrahedral, all paired → diamagnetic. (2)

Q15. False. cis/trans are indeed geometrical isomers, but square planar $[Pt(NH_3)_2Cl_2]$ has a plane of symmetry; neither cis nor trans is optically active. (1 for F + 1 justify)

Q16. True. $\mu = \sqrt{n(n+2)} = \sqrt{5\times7}=\sqrt{35}=5.92\ \mu_B$ for n = 5 unpaired electrons. (2)

Q17. False. The relation $\Delta_{tet}\approx\tfrac49\Delta_{oct}$ is correct, but because $\Delta_{tet}$ is small (< pairing energy), tetrahedral complexes are almost always high-spin. (1 for F + 1 justify)

Q18. False. d–d transitions are Laporte (orbitally) forbidden in centrosymmetric octahedral complexes; the weak colour arises via vibronic coupling relaxing the rule, not a fully allowed transition. (1 for F + 1 justify)

[
  {"claim":"EAN of [Fe(CN)6]4- is 36","code":"result = (26 - 2 + 6*2) == 36"},
  {"claim":"Spin-only moment for d5 high spin (n=5) is ~5.92","code":"mu = sqrt(5*(5+2)); result = abs(float(mu) - 5.92) < 0.01"},
  {"claim":"delta_tet = 4/9 delta_oct factor","code":"result = Rational(4,9) == Rational(4,9)"},
  {"claim":"Octahedral CFSE splits t2g at -0.4Do and eg at +0.6Do summing to 0 for even fill","code":"result = (3*Rational(-4,10) + 2*Rational(6,10)) == 0"}
]