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In a certain octahedral complex ion having the formula [Fe(Cl)(CN)4(O2)]⁴-, the crystal field splitting results in the electronic configuration t2g⁶ eg⁰ for the metal. Which of the following statements about this complex ion is/are correct?

1. The complex is paramagnetic
2. The O-O bond length in the coordinated O2 ligand is greater than that in free O2 molecule
3. The IUPAC name of the complex is chloridotetracyanido(dioxygen)ferrate(II) ion
4. The complex exhibits both geometrical and optical isomerism

Correct answer: The O-O bond length in the coordinated O2 ligand is greater than that in free O2 molecule

Solution

t2g⁶ eg⁰ configuration has all electrons paired, so the complex is diamagnetic — option (A) is wrong. Back-donation from Fe into the pi-antibonding orbital of O2 weakens and lengthens the O-O bond compared to free O2 — option (B) is correct. Naming and isomerism statements have errors in the given options.

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