UPSC Civil Services (Mains), Chemistry optional · 2026 · Paper I

Q5(g). Ferrocene, the sandwich compound

Question

Why is ferrocene termed as a sandwich compound? Explain its structure.

In ferrocene, [Fe(η⁵-C₅H₅)₂], an Fe(II) atom lies between two parallel, planar cyclopentadienyl rings, like the filling between two slices of bread.

the two C₅H₅ rings are parallel and the Fe atom sits on the common fivefold axis midway between them. All ten carbon atoms are bonded equally to Fe (η⁵, hapticity 5), with Fe–C = 206 pm.
Each ring is an aromatic C₅H₅⁻ (6 π electrons) donating to Fe 3d, 4s, 4p. Fe(II) d⁶ + 2 × 6 = 18 electrons: diamagnetic and very stable; oxidised to blue [Fe(C₅H₅)₂]⁺.
gas phase eclipsed (D₅ₕ), about 4 kJ mol⁻¹ below staggered (D_(5d), reported in early X-ray work); the rings rotate easily at room temperature.

The full model answer

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Sandwich: the two C₅H₅ rings are parallel and the Fe atom sits on the common fivefold axis midway between them. All ten carbon atoms are bonded equally to Fe (η⁵, hapticity 5), with Fe–C = 206 pm.
Each ring is an aromatic C₅H₅⁻ (6 π electrons) donating to Fe 3d, 4s, 4p. Fe(II) d⁶ + 2 × 6 = 18 electrons: diamagnetic and very stable; oxidised to blue [Fe(C₅H₅)₂]⁺.
Ring orientation: gas phase eclipsed (D₅ₕ), about 4 kJ mol⁻¹ below staggered (D_(5d), reported in early X-ray work); the rings rotate easily at room temperature.

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