Abstract
Iron carbonyls are known to form 18-electron complexes like Fe(CO)5, Fe2(CO)9, and Fe3(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]4s23d6 (S = 2) → [Ar]4s03d8 (S = 0) excitation energy to stabilize a closed-shell Fe(CO)4 with a d8-configuration. Strong σ(4CO) → Fe (dx2-y2) bonding along with Fe(dxz, dyz) and Fe(dxy) → π (CO)4* back-bonding assists the formation of square-planar Fe(CO)4 under pressure. Compression progressively flattens and destabilizes the ambient pressure C2v structure of Fe(CO)4, and beyond 2 GPa, it undergoes a sharp C2v → D4h transition with ΔVunit-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.
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