Which of the following pairs of transition metal ions are the stronge…

The correct answer is Option (4) → Mn³⁺ and Fe³⁺

An ion is a strong oxidising agent when it readily takes an electron, that is, when its E°(M³⁺/M²⁺) value is high and positive. E°(Mn³⁺/Mn²⁺) = +1.57 V is very high because the change gives \( \ce{Mn^2+} \) the stable half-filled d⁵ configuration. E°(Fe³⁺/Fe²⁺) = +0.77 V, so \( \ce{Fe^3+} \) is also an oxidant. This is the only pair in which both ions are oxidising agents.

• Option (1) Fe²⁺ and Mn²⁺ — Incorrect. These are the reduced forms. \( \ce{Fe^2+} \) is a reducing agent, and \( \ce{Mn^2+} \) (d⁵) is very stable and has no tendency to gain an electron.

• Option (2) Cu²⁺ and Ti²⁺ — Incorrect. \( \ce{Ti^2+} \) is a strong reducing agent that liberates hydrogen from dilute acid, and \( \ce{Cu^2+} \) is only a weak oxidant.

• Option (3) Ni²⁺ and Cr³⁺ — Incorrect. \( \ce{Cr^3+} \) (stable half-filled t_2g³) and \( \ce{Ni^2+} \) are both stable ions and are not good oxidising agents.

NCERT (2026-27 edition):
Chemical Reactivity and E° Values – “… An examination of the E° values for the redox couple M³⁺/M²⁺ (Table 4.2) shows that Mn³⁺ and Co³⁺ ions are the strongest oxidising agents in aqueous solutions. The ions Ti²⁺, V²⁺ and Cr²⁺ are strong reducing agents and will liberate hydrogen from a dilute acid.”