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Cr²+ acts as a reducing agent primarily because:

1. Its configuration changes from d⁴ to d³, achieving a half-filled t2g level in octahedral field.
2. Cr²+ gains an electron to reach the d⁵ configuration.
3. Cr²+ loses an electron to reach the d⁵ configuration.
4. Its configuration changes from d⁴ to d³, achieving a half-filled d subshell.

Correct answer: Its configuration changes from d⁴ to d³, achieving a half-filled t2g level in octahedral field.

Solution

Cr²+ has [Ar]3d⁴ configuration. It readily loses one electron to form Cr³+ ([Ar]3d³). In an octahedral crystal field, d³ corresponds to t2g³ eg⁰ — a half-filled t2g set, which is extra stable due to exchange energy. This thermodynamic driving force makes Cr²+ a good reducing agent.

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