UPSC Combined Geo-Scientist, Mains · 2018 · Mains Paper I
Q1(f). Predicting carbonyl numbers
Question
What is 18-electron rule? Using the 18-electron rule predict the n value for the following compounds: (i) [η⁶(C₆H₆)Cr(CO)ₙ] (ii) [η⁴(C₄H₄)₂Mn(Cl)(CO)ₙ].
Answer in brief
An 18-electron metal centre fills its nine valence orbitals. The predicted values are (i) n=3 and (ii) n=1.
Key points
- In neutral counting, chromium supplies 6 electrons and η⁶ benzene 6. Therefore 6+6+2n=18, giving n=3.
- Manganese supplies 7 electrons, the two η⁴ rings supply 2(4)=8, chloride supplies 1, and each CO supplies 2. Thus 7+8+1+2n=18, giving n=1.
- The rule is an electron-counting stability guideline, especially useful for low-oxidation-state organometallic compounds; it is not a universal requirement for all complexes.
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.
CoversPredicting carbonyl numbersElectron counting
All parts of Question 1
- 1(a) Lithium hydride5 marks
- 1(b) Coordination nomenclature5 marks
- 1(c) Werner valencies and isomers5 marks
- 1(d) Bronsted and Lewis acids5 marks
- 1(e) Halogen displacement5 marks
- 1(f) Predicting carbonyl numbers5 marks
- 1(g) Radiocarbon age5 marks
- 1(h) Alkali-ion selectivity of resins5 marks
- 1(i) Borazine and benzene5 marks
- 1(j) Anomalous lanthanide radii5 marks