Q1(h). Terminal and bridging carbonyl bands

Answer in brief

The 1800 cm⁻¹ band is assigned to bridging CO; the ~1980 and 2020 cm⁻¹ bands are coupled terminal-CO stretching modes.

Key points

• The low-frequency band indicates μ₂-CO bridges. A suitable form is Cp(CO)Fe(μ-CO)₂Fe(CO)Cp with an Fe–Fe bond.
• Each terminal CO is bound to one Fe; each bridging CO is bonded through carbon to both metals.
• Metal-to-CO π back-donation populates CO π* orbitals and lowers the C–O force constant. Bridging carbonyl stretches occur below terminal stretches.
• The two terminal C–O oscillators give symmetry-adapted combinations and hence distinct stretching frequencies near 1980 and 2020 cm⁻¹.