UPSC Civil Services (Mains)
Chemistry optional
- 2 papers
- 250 marks each
- 3 h per paper
- 5 of 8 questions, Q1 and Q5 compulsory
Paper I covers physical and inorganic chemistry.
Coordination chemistry: every question, 2021 to 2025
28 parts worth 300 marks have been set on this unit in UPSC CSE Chemistry Paper I from 2021 to 2025. Each one opens on its own page with the question, the answer in brief and the key points.
(i) Bonding in transition metal complexes. Valence bond theory, crystal field theory and its modifications; applications of theories in the explanation of magnetism and electronic spectra of metal complexes. (ii) Isomerism in coordination compounds; IUPAC nomenclature of coordination compounds; stereochemistry of complexes with 4 and 6 coordination numbers; chelate effect and polynuclear complexes; trans effect and its theories; kinetics of substitution reactions in square-planar complexes; thermodynamic and kinetic stability of complexes. (iii) EAN rule, synthesis, structure and reactivity of metal carbonyls; carboxylate anions, carbonyl hydrides and metal nitrosyl compounds. (iv) Complexes with aromatic systems, synthesis, structure and bonding in metal olefin complexes, alkyne complexes and cyclopentadienyl complexes; coordinative unsaturation, oxidative addition reactions, insertion reactions, fluxional molecules and their characterization; compounds with metal-metal bonds and metal atom clusters.
Explained on the blog: Crystal field theory: why [CoF₆]³⁻ is paramagnetic but [Co(CN)₆]³⁻ is diamagnetic (asked in 2023 and 2024)
Notes: Crystal field theory (full notes) · short notes
Notes: Werner theory, isomerism, IUPAC names and VBT of complexes (full notes) · short notes
Notes: Organometallics: 18-electron rule, oxidative addition, Wilkinson's catalyst (full notes) · short notes
Notes: Metal carbonyls, nitrosyls, 18-electron rule and metal clusters (full notes) · short notes
Notes: Substitution in complexes: labile and inert, trans effect, stability constants (full notes) · short notes
Notes: Electronic spectra of complexes: term symbols, selection rules, charge transfer (full notes) · short notes
2025
- Q5(f) Which complex has the larger Δₒ10 marks Answered
- Q6(a) Rigid or fluxional: an allyl palladium complex10 marks Answered
- Q7(c) Stereoisomers of a doubly bridged dicobalt complex10 marks Answered
- Q7(e) Substitution in [PtCl₃(C₂H₄)]⁻ and the trans effect10 marks Answered
- Q8(e) Two oxidative additions of methyl iodide10 marks Answered
2024
- Q5(c) IUPAC names of five complexes10 marks Answered
- Q5(d) Zeise's salt: synthesis and structure10 marks Answered
- Q7(a) Splitting of d orbitals in a square-planar field10 marks Answered
- Q7(b)(i) M–C and C–O bond orders in Ni(CO)₄, [Co(CO)₄]⁻ and [Fe(CO)₄]²⁻10 marks Answered
- Q7(b)(ii) Identify A, B and C5 marks Answered
- Q7(c)(i) [CoF₆]³⁻ paramagnetic, [Co(CN)₆]³⁻ diamagnetic10 marks Answered
- Q7(c)(ii) Structures and magnetic behaviour of Co₂(CO)₈15 marks Answered
2023
- Q5(e) Trans effect: A and B from [PtCl₄]²⁻10 marks Answered
- Q6(b) Stereoisomers of [Co(CN)₂(H₂O)₂(NH₃)₂]⁺10 marks Answered
- Q6(c) IUPAC names of five complexes10 marks Answered
- Q7(d) Electronic spectrum of [Ni(OS(CH₃)₂)₆]²⁺10 marks Answered
- Q8(a) Acetylferrocene and ferrocenecarboxylic acid from ferrocene10 marks Answered
- Q8(b) Spin-only magnetic moments of four complexes10 marks Answered
2022
- Q5(d) [Ni(CO)₄] and [Ni(CN)₄]⁴⁻: names, structures and shapes10 marks Answered
- Q6(a) Metal–metal bonds in Cp₂Fe₂(CO)₄; structure of Bi₅³⁺10 marks Answered
- Q6(c) Neutral mixed sandwich compound of a d⁶ centre10 marks Answered
- Q6(e) Origin of the yellow colour of aqueous FeCl₃10 marks Answered
- Q7(a) An optically active square-planar complex10 marks Answered
- Q7(e) Magnetic behaviour of copper(II) acetate and [Cu(CN)₄]³⁻10 marks Answered
- Q8(a) The more stable linkage isomer of a cyanide-bridged complex10 marks Answered
2021
- Q5(d) Geometrical isomers and stereoisomers of M(AB)b₂c₂10 marks Answered
- Q7(b) Bonding in two fluxional molecules from variable-temperature ¹H NMR20 marks Answered
- Q8(c) Linear and bent nitrosyl in [Ru(PPh₃)₂Cl(NO)₂]⁺20 marks Answered
UPSC Civil Services (Main) Chemistry Paper I: Model Answers 2021 to 2025
- All eight questions of each paper, with the question printed above its answer
- Question choice and time plan for each paper
- Topic-wise weightage and a topic-wise index