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Two coordination complexes both have the electronic configuration t2g⁶ eg⁰ for the central metal ion (strong field, deltaₒ > P). One complex is [Fe(CN)6]⁴- and the other is [Co(CN)6]³-. Which of the following statements is correct for both complexes?

1. Both act as oxidising agents in acidic medium
2. Both are diamagnetic and form coloured compounds
3. O2 is used in the preparation of both
4. The central atom of the anion in both has d³s hybridisation

Correct answer: Both are diamagnetic and form coloured compounds

Solution

t2g⁶ eg⁰ means all 6 d-electrons are paired in the t2g orbitals: zero unpaired electrons, so both complexes are diamagnetic. Both are also coloured due to MLCT (metal-to-ligand charge transfer) transitions, even though d-d transitions are forbidden for fully filled t2g. Option A is wrong because [Fe(CN)6]⁴- is a reducing agent. Option C is wrong. Option D is wrong as CN⁻ ligands cause d²sp³ hybridisation (inner orbital), not d³s.

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