UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper I
Q1(c). Trans effect: Niᴵᴵ, Ptᴵᴵ, Pdᴵᴵ
Question
Arrange the following metals in descending order of their trans-effect with proper justification: Niᴵᴵ, Ptᴵᴵ, Pdᴵᴵ.
Answer in brief
Trans effect: Ptᴵᴵ > Pdᴵᴵ > Niᴵᴵ.
Key points
- Orbital size 5d > 4d > 3d: larger, more diffuse 5d orbitals give the best overlap with π* of T, strongest π-back-bonding in the transition state.
- σ trans influence: Pt–L bonds are most covalent; a strong σ donor T weakens the trans M–X bond most for Pt (polarisation model, Grinberg).
- Lability: rates Ni : Pd : Pt ≈ 10⁷ : 10⁵ : 1. Labile Ni(II) hardly discriminates; inert Pt(II) shows the largest trans-directing effect.
- Square-planar d⁸ substitution is associative through a five-coordinate trigonal-bipyramidal intermediate.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coverstrans effectsquare planar complexesPt(II)Grinberg polarisationcisplatin synthesis
All parts of Question 1
- 1(a) Defect in silver chloride5 marks
- 1(b) Isomerism in [(NH₃)₅CoONO]Cl₂ and [(NH₃)₅CoNO₂]Cl₂5 marks
- 1(c) Trans effect: Niᴵᴵ, Ptᴵᴵ, Pdᴵᴵ5 marks
- 1(d) Phenolphthalein in dilute and in concentrated alkali5 marks
- 1(e) Solubility product of silver chromate5 marks
- 1(f) Colour change interval of diphenylamine5 marks
- 1(g) Eriochrome Black T and calcium5 marks
- 1(h) Tendency to form metal clusters5 marks
- 1(i) Complete the nuclear equations5 marks
- 1(j) First ionisation enthalpy of 5d elements5 marks