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

Abstract

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]4f145d66s2 (S=2) → [Xe]4f145d8 (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.