Paper I — Q8
(a) Justify that the interhalogen compound BrF₃ acts as an aprotic solvent and undergoes acid-base and neutralization reactions…
Justify that the interhalogen compound BrF₃ acts as an aprotic solvent and undergoes acid-base and neutralization reactions by giving examples. 10 marks
The observed magnetic moments of lanthanide ions in general differ from observed magnetic moments of first row transition metal ions. Explain by giving reason(s). Identify the lanthanide ions having magnetic moments corresponding to spin-only value, and those which are diamagnetic. 20 marks
In the coordination compound [Ru(PPh₃)₂Cl(NO)₂]⁺, one NO ligand bonds linearly while the other is bent. Explain the different modes of bonding of NO ligands in this molecule and expected M—N bond orders. 20 marks
हिंदी में प्रश्न पढ़ें
उदाहरण देते हुए उचित सिद्ध कीजिए कि अंतरहैलोजन यौगिक BrF₃, ऐप्रोटिक विलायक के रूप में क्रिया करता है और अम्ल-क्षारक तथा निष्प्रभावन/उदासीनिकरण अभिक्रियाओं को सहता है/से गुजरता है। (10 अंक)
लैन्थेनाइड आयनों के प्रेक्षित चुंबकीय आघूर्ण सामान्य तौर पर प्रथम पंक्ति संक्रमण धातु आयनों के प्रेक्षित चुंबकीय आघूर्ण से अलग हैं। कारण सहित इसकी व्याख्या कीजिए। ऐसे लैन्थेनाइड आयनों को पहचानिए, जिनके चुंबकीय आघूर्ण केवल प्रचक्रण के मान के अनुरूप हैं और जो प्रतिचुंबकीय हैं। (20 अंक)
उपसहसंयोजन यौगिक [Ru(PPh₃)₂Cl(NO)₂]⁺ में एक NO लिगैंड (संलगी) बंध रैखिकतः है जबकि दूसरा बंकित है। इस अणु में NO लिगैंड की अलग-अलग आबंधक विधा और अनुमानित M—N आबंध क्रमों की व्याख्या कीजिए। (20 अंक)
Model answer
Drafted with AI by UPSC Answer Check against this question's marking rubric, to the expected length. UPSC does not publish answers for Mains — this is one way to score well, not an official key. How we build rubrics and answers.
(a) BrF₃ as an aprotic solvent BrF₃ is a polar liquid (m.p. 8.8 °C, b.p. 126 °C) with no hydrogen, so it cannot donate protons; it is an aprotic solvent, but self-ionises by fluoride transfer, 2BrF₃ ⇌ BrF₂⁺ + BrF₄⁻, with BrF₂⁺ as the solvo-acid and BrF₄⁻ as the solvo-base. Hence acids are F⁻ acceptors and bases are F⁻ donors. Acid: SbF₅ + BrF₃ → [BrF₂]⁺[SbF₆]⁻ (also SnF₄, AuF₃). Base: KF + BrF₃ → K⁺[BrF₄]⁻ (also AgF, NOF). Neutralisation: [BrF₂]⁺[SbF₆]⁻ + K⁺[BrF₄]⁻ → KSbF₆ + 2BrF₃, which is salt plus solvent, as H⁺ + OH⁻ → H₂O in water. BrF₃ is also a strong fluorinating and oxidising solvent, converting metals and oxides to fluorides, e.g., silver gives Ag⁺[BrF₄]⁻, and conducts electricity because of ionisation.
(b) Magnetic moments of lanthanide ions In first-row transition metal ions, the 3d electrons are exposed to the ligand field, which quenches the orbital angular momentum and makes spin-only values μ = √[n(n+2)] BM usually valid. In lanthanide ions the 4f electrons lie deep inside, shielded by the filled 5s² and 5p⁶ shells, so the ligand field is negligible and the orbital contribution is not quenched. Spin–orbit coupling (about 1,000–3,000 cm⁻¹) is large compared with kT, so J is a good quantum number in the Russell–Saunders scheme, and μ = g_J√[J(J+1)], with g_J = 1 + [J(J+1) + S(S+1) − L(L+1)]/2J(J+1). This formula fits most Ln³⁺ ions (e.g., Pr³⁺ 3.6, Dy³⁺ 10.6 BM), apart from Sm³⁺ and Eu³⁺, where low-lying excited J levels are thermally populated, so that the observed values (about 1.5 and 3.4 BM) exceed the calculated ones. Spin-only values are observed for ions with L = 0: Gd³⁺ (4f⁷, ⁸S₇/₂, 7.94 BM) and Eu²⁺ (4f⁷), as the orbital contribution vanishes. Diamagnetic ions have no unpaired electrons: La³⁺ and Ce⁴⁺ (4f⁰), and Lu³⁺ and Yb²⁺ (4f¹⁴).
(c) NO ligands in [Ru(PPh₃)₂Cl(NO)₂]⁺ NO has one electron in its π* orbital and can coordinate in two ways. Linear nitrosyl (M–N–O ≈ 175–180°): treated as NO⁺ (isoelectronic with CO), a 2-electron σ-donor through N and a strong π-acceptor, since Ru dπ electrons flow into the empty NO π* orbital. The M–N bond has multiple character, M=N=O ↔ M≡N⁺–O, order about 2–3 (short Ru–N, about 1.74 Å), and the N–O stretch is high (about 1800–1950 cm⁻¹). (In the covalent count, linear NO is a 3-electron donor.) Bent nitrosyl (M–N–O ≈ 120–140°): treated as NO⁻ (isoelectronic with O₂), whose N atom is sp²-hybridised with a lone pair donated to Ru (2-electron σ-donor ionic count, 1-electron in the neutral count), giving a M–N single bond, order about 1 (longer Ru–N, about 1.86 Å), with N–O lower at about 1600–1720 cm⁻¹ and little π-back-donation. With Cl⁻, NO⁺ and NO⁻ the ruthenium is Ru(II), d⁶. In the {M(NO)₂} Enemark–Feltham notation, the two NO ligands adopt different geometries to balance the electron requirements of the metal.
How this answer will be evaluated
Framework: Concept > Structure or mechanism > Reasoning > Result
What earns full marks: Complete mechanisms, correct examples, precise bond orders, clear reasoning throughout
Evaluation rubric
Each sub-part is marked on its own, against the marks and word limit printed on the paper.
- (a) Explain BrF3 as aprotic solvent and its acid-base/neutralization behavior with examples. 10 marks
justify— claim → 3-4 reasons → evidence → conclusion
Must cover
- Define BrF3 as aprotic (no protons to donate)
- Explain autoionization of BrF3 (2BrF3 ⇌ BrF2+ + BrF4-)
- Give example of acid-base reaction in BrF3
- Give example of neutralization reaction in BrF3
Loses marks
- Confusing aprotic with non-polar solvent
- Failing to show autoionization equation
- Giving examples from aqueous system instead of BrF3
Earns more
- Mention Lewis acid-base character of BrF3
- Show electron pair donation/acceptance mechanism
- Compare with water autoionization
- Mention specific compounds reacting in BrF3
Extra mark
- Draw Lewis structures showing electron pairs
- Mention industrial applications of BrF3 as solvent
- (b) Explain why lanthanide magnetic moments differ from transition metals; identify spin-only and diamagnetic ions. 20 marks
explain— definition/context → points in order → small example → short close
Must cover
- Explain spin-orbit coupling in lanthanides (4f orbitals)
- Contrast with transition metals (3d, weaker spin-orbit)
- Identify lanthanide ions with spin-only magnetic moments (L = 0 ions: Gd3+ and Eu2+, f7, about 7.94 BM)
- Identify diamagnetic lanthanide ions (La3+, Ce4+ 4f0; Lu3+, Yb2+ 4f14)
Loses marks
- Using only spin-only formula for lanthanides
- Failing to identify specific diamagnetic ions
- Confusing 4f with 3d orbital behavior
Earns more
- Mention Russell-Saunders coupling scheme
- Show formula μ = g√J(J+1) for lanthanides
- Explain why 4f orbitals are shielded
- List specific ions with their magnetic moments
Extra mark
- Provide table of lanthanide ions with μ values
- Mention temperature dependence of magnetic moments
- (c) Explain linear vs bent NO bonding in [Ru(PPh3)2Cl(NO)2]+ and expected M-N bond orders. 20 marks
explain— definition/context → points in order → small example → short close
Must cover
- Explain linear NO as NO+ (nitrosonium) with 2-electron donation
- Explain bent NO as NO- (N sp2 lone pair donation; 2-electron ionic count, 1-electron neutral count)
- State M-N bond order for linear NO (about 2-3, strong back-donation)
- State M-N bond order for bent NO (about 1, single bond)
Loses marks
- Treating both NO ligands identically
- Failing to specify bond orders for each type
- Confusing NO+ and NO- electron donation
Earns more
- Show oxidation state of Ru (Ru(II), d6) with NO+ and NO-
- Explain back-bonding from Ru to NO π* orbitals
- Mention Enemark-Rohlfson notation for NO
- Draw structures showing linear vs bent geometry
Extra mark
- Provide IR stretching frequency data for both NO types
- Mention specific bond lengths from crystallographic data
What "Explain" is asking you to do
Make the working of something clear — what sets it off, what follows from what, and what it produces. Explain is the Commission's mechanism word: it dominates the technical papers and the “explain why” stems, where the marks sit in the causal chain and not in the label.
Structure that answers it
State what it is → the initiating condition → the chain of cause, step by step → an instance where it plays out → what the chain produces
Where marks are lost
Describing what something looks like instead of why it works that way. Naming the stages without linking them reads as description too.
Practice this exact question
Write your answer and it is marked point by point against the model answer above — what you covered, what you missed, what you got wrong.
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