Pressure Induced Alteration and Stabilization of Intermolecular Stacking in Square-planar Osmium tetracarbonyl

Osmium carbonyls are well known to form stable 18-electron complexes like Os(CO)₅ , Os₂ (CO)₉ and Os₃ (CO)₁₂ having both bridging and terminal carbonyls. For osmium tetra-carbonyl, Os(CO)₄ solid-state packing significantly alters the ground-state structure. The gas-phase stable see-saw geometry converts to a square-planar structure in solid state. Highly efficient intermolecular stacking between Os(CO)₄ units assists this transformation. Each Os(CO)₄ molecule is stacked in a staggered orientation with respect to each other. Pressure induces a [Xe]4f¹⁴ 5d⁶ 6s² (S=2)→[Xe]4f¹⁴ 5d⁸ (S=0) electronic transition in osmium stabilize a square planar osmium tetra-carbonyl. Under the influence of isotropic pressure, the molecules not only come closer to each other but their relative orientations also get significantly altered. Calculations show that at P=1 GPa and above, the eclipsed orientation for the intermolecular stacking gets preferred over the staggered form. The staggered→eclipsed intermolecular stacking orientation under pressure is shown to be controlled by London dispersion interactions.