UPSC Combined Geo-Scientist, Mains · 2022 · Mains Paper I
Q1(c). Square-planar stereoisomers
Question
How many stereoisomers are possible for the square-planar complexes (i) Ma₂bc and (ii) Mabcd (a, b, c and d are unidentate ligands)? Draw the structures.
Answer in brief
Ma₂bc has two geometrical isomers; Mabcd has three. For ordinary achiral point ligands these planar arrangements have no optical isomers.
Key points
- For Ma₂bc, the two identical a ligands may be adjacent (cis) or opposite (trans).
- For Mabcd, fix a at one site. Its trans partner can be b, c or d; each choice defines one distinct arrangement.
- The corresponding trans-pair sets are a/b with c/d, a/c with b/d, and a/d with b/c.
- Rotating the whole square creates no new isomer. The molecular plane is a mirror plane in this idealised point-ligand model.
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.
Coverssquare planar isomerismgeometrical isomerscis-transMabcd isomers
All parts of Question 1
- 1(a) Minerals and framework structures5 marks
- 1(b) Ground terms by L-S coupling5 marks
- 1(c) Square-planar stereoisomers5 marks
- 1(d) Carbonate in carbonic acid5 marks
- 1(e) Chloride from silver chloride5 marks
- 1(f) Iron percentage by permanganate5 marks
- 1(g) Masking copper to determine nickel5 marks
- 1(h) Carbonyl ligand counts5 marks
- 1(i) Carbon-14 dating of wood5 marks
- 1(j) Oxidising power across the series5 marks