UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper I

Q1(b). Isomerism in [(NH₃)₅CoONO]Cl₂ and [(NH₃)₅CoNO₂]Cl₂

Question

Which isomerism do [(NH₃)₅CoONO]Cl₂ and [(NH₃)₅CoNO₂]Cl₂ complexes show and why?

Linkage isomerism: same formula, but NO₂⁻ is bonded through O (nitrito, red) in one and through N (nitro, yellow) in the other.

NO₂⁻ has a lone pair on N and on each O: two possible donor atoms (ambidentate).
Nitrito forms first (kinetic product); on standing or warming it changes to the stable nitro form. UV light reverses it.
Proof by IR: nitro 1430 and 1315 cm⁻¹; nitrito 1460 and 1065 cm⁻¹.
First example of linkage isomerism (Jørgensen); SCN⁻ and CN⁻ show the same type.

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.

Coverslinkage isomerismnitro nitritoambidentate ligandpentaamminecobalt(III)Jørgensen