Catching Wyckoff’s Phantom: The Peculiar Crystal Structure of Orthorhombic Cs[ICl ₂ ]

The crystal structure of the thus far elusive orthorhombic modification of Cs[ICl ₂ ] ( Pnma , Z  = 4, a  = 1042.2(1) pm, b  = 647.35(5) pm, c  = 870.38(8) pm and V  = 587.21(9) Å ³ ) is reported. In contrast to the structure of the known trigonal modification ( R m ), the [ICl ₂ ] ⁻ anion shows a significant I─Cl bond length asymmetry, similar to other cesium trihalides. We used quantum chemical calculations (DFT) to get further insights in the molecular electrostatic potential surfaces of the [ICl ₂ ] ⁻ anion found in the new polymorph, as well as the gas‐phase optimized structure as a reference. Additionally, quantum theory of atoms in molecules and natural bond orbital analyses were performed to quantify the strength of the observed noncovalent interactions and the orbital contributions and explain the deviations found between the anion in the novel polymorph and the so far reported species. Our findings show that the orthorhombic modification most likely is a metastable modification which can be described as a transient Cl ⁻ ···I–Cl intermediate toward a symmetric [Cl–I–Cl] ⁻ anion.