UPSC Combined Geo-Scientist, Mains · 2022 · Mains Paper I

Q1(h). Carbonyl ligand counts

Question

Consider the 18-electron rule and determine the value of n for the following compounds: (i) K₂Fe(CO)ₙ and (ii) MnCl(CO)ₙ.

(i) n = 4; (ii) n = 5.

K₂Fe(CO)ₙ contains [Fe(CO)ₙ]²⁻. CO is neutral, so Fe has formal oxidation state −2 and d count 8 − (−2) = 10.
Each CO donates two electrons: 10 + 2n = 18, hence n = 4. The anion is tetrahedral [Fe(CO)₄]²⁻.
In neutral MnCl(CO)ₙ, Cl⁻ gives Mn(I), d⁶. Count chloride as a two-electron donor in the ionic method.
6 + 2 + 2n = 18, hence n = 5. Do not mix ionic and neutral counting conventions.

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.

Covers18-electron rulemetal carbonylselectron countingCollman's reagent K2Fe(CO)4MnCl(CO)5