UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper I
Q1(h). Tendency to form metal clusters
Question
Which metals of the following have the greatest tendency to form metal clusters? Justify your answer. (i) V, Nb, Ta (ii) Zr, V, Nb (iii) Cr, Mo, Tc (iv) Nb, Mo, Tc
Answer in brief
(i) Ta (ii) Nb (iii) Mo (Tc almost equal) (iv) Nb: the heavier 4d/5d metals with the largest enthalpy of atomisation win.
Key points
- Criterion: high ΔHatom means strong M–M bonding.
- Down a group 3d ≪ 4d < 5d: diffuse 4d/5d orbitals overlap well; 3d orbitals are too contracted → weak M–M bonds.
- Mid-series: groups 5–7 have enough d electrons for M–M bonding without filling antibonding levels.
- Low oxidation state keeps d orbitals expanded, e.g. [Nb₆Cl₁₂]²⁺, [Ta₆Cl₁₂]²⁺, [Mo₆Cl₈]⁴⁺, [Tc₂Cl₈]³⁻.
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.
Coversmetal clustersmetal–metal bondingenthalpy of atomisation4d and 5d metals[Mo2Cl8]4- quadruple bond
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