Unraveling the Bonding Complexity of Polyhalogen Anions: High-Pressure Synthesis of Unpredicted Sodium Chlorides Na(2)Cl(3) and Na(4)Cl(5) and Bromide Na(4)Br(5)

[Image: see text] The field of polyhalogen chemistry, specifically polyhalogen anions (polyhalides), is rapidly evolving. Here, we present the synthesis of three sodium halides with unpredicted chemical compositions and structures (tP10-Na(2)Cl(3), hP18-Na(4)Cl(5), and hP18-Na(4)Br(5)), a series of isostructural cubic cP8-AX(3) halides (NaCl(3), KCl(3), NaBr(3), and KBr(3)), and a trigonal potassium chloride (hP24-KCl(3)). The high-pressure syntheses were realized at 41–80 GPa in diamond anvil cells laser-heated at about 2000 K. Single-crystal synchrotron X-ray diffraction (XRD) provided the first accurate structural data for the symmetric trichloride Cl(3)(–) anion in hP24-KCl(3) and revealed the existence of two different types of infinite linear polyhalogen chains, [Cl](∞)(n–) and [Br](∞)(n–), in the structures of cP8-AX(3) compounds and in hP18-Na(4)Cl(5) and hP18-Na(4)Br(5). In Na(4)Cl(5) and Na(4)Br(5), we found unusually short, likely pressure-stabilized, contacts between sodium cations. Ab initio calculations support the analysis of structures, bonding, and properties of the studied halogenides.