More practice questions

  1. Q4. Explain why [Co(NH3)6]3+ is an inner orbital complex whereas [Ni(NH3)6]2+ is an outer orbital complex. (NCERT Intext Question 5.8.)

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    Co3+ is 3d6: with NH3 the electrons pair up in three 3d orbitals, leaving two 3d orbitals empty for d2sp3, an inner orbital complex. Ni2+ is 3d8: eight electrons cannot leave two 3d orbitals empty, so the outer 4d orbitals are used, sp3d2, an outer orbital complex (NCERT answer, page 140).

  2. Q5. The hexaaquamanganese(II) ion contains five unpaired electrons, while the hexacyanido ion contains only one. Explain using crystal field theory. (NCERT Intext Question 5.10.)

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    Mn2+ is d5. H2O is a weak field ligand, Δo < P, so the electrons spread out as t2g3 eg2: five unpaired. CN− is a strong field ligand, Δo > P, so all five go into t2g, t2g5 eg0: one unpaired (reasoned from NCERT page 132 and the spectrochemical series).

  3. Q6. Discuss the nature of bonding in (i) [Fe(CN)6]4−, (ii) [FeF6]3−, (iii) [Co(C2O4)3]3− and (iv) [CoF6]3− on the basis of valence bond theory. (NCERT Exercise 5.15.)

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    (i) Fe2+, 3d6, paired by CN−: d2sp3, inner orbital, octahedral, diamagnetic. (ii) Fe3+, 3d5, unpaired: sp3d2, outer orbital, octahedral, five unpaired electrons. (iii) Co3+, 3d6, paired: d2sp3, inner orbital, diamagnetic. (iv) Co3+, 3d6 as in the free ion, four unpaired: sp3d2, outer orbital, four unpaired electrons (reasoned; (ii)–(iv) as NCERT states on page 130).

  4. Q7. [Cr(NH3)6]3+ is paramagnetic while [Ni(CN)4]2− is diamagnetic. Explain why. (NCERT Exercise 5.19.)

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    Cr3+ is 3d3: two 3d orbitals are free for d2sp3 without any pairing, so the three electrons stay unpaired, μ = √15 = 3.87 BM. Ni2+ in [Ni(CN)4]2− is 3d8 with all electrons paired for dsp2 (NCERT page 130; reasoned).

  5. Q8. Which has the highest magnetic moment: [Cr(H2O)6]3+, [Fe(H2O)6]2+ or [Zn(H2O)6]2+? (NCERT Exercise 5.29.)

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    Cr3+, d3: 3 unpaired, 3.87 BM. Fe2+, d6 with weak field H2O: t2g4 eg2, 4 unpaired, √24 = 4.90 BM. Zn2+, d10: 0. So [Fe(H2O)6]2+ (reasoned).

  6. Q9. Why are [Fe(CN)6]4− and [Fe(H2O)6]2+ different colours in dilute solution? (NCERT Exercise 5.21.)

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    Both contain Fe2+, d6, but CN− is a far stronger field ligand than H2O (NCERT page 132). Their Δo values differ (t2g6 against t2g4 eg2), so they absorb different light and show different complementary colours (reasoned).