Compression Produces a Square-Planar Iron Tetracarbonyl.

Iron carbonyls are known to form 18-electron complexes like Fe(CO) 5 , Fe 2 (CO) 9 , and Fe 3 (CO) 12 having terminal or bridged Fe-CO bonding. Based on genetic algorithm-assisted density functional theory (DFT) calculations, it is predicted that at pressures above 2 GPa, iron tetracarbonyl, Fe(CO) 4 , attains a square-planar geometry with a 16-electron count. Compression overcomes the [Ar]4s 2 3d 6 ( S = 2) → [Ar]4s 0 3d 8 ( S = 0) excitation energy to stabilize a closed-shell Fe(CO) 4 with a d 8 -configuration. Strong σ(4CO) → Fe (d x 2 - y 2 ) bonding along with Fe(d xz , d yz ) and Fe(d xy ) → π (CO) 4 * back-bonding assists the formation of square-planar Fe(CO) 4 under pressure. Compression progressively flattens and destabilizes the ambient pressure C 2v structure of Fe(CO) 4 , and beyond 2 GPa, it undergoes a sharp C 2v → D 4h transition with Δ V unit-cell = 2.1% and trans -θ(OC-Fe-CO) = 180°. Realizing a square-planar geometry in a four-coordinated Fe-carbonyl complex shows the rich prospects of the new chemistry under pressure.