From Polyanions to Infinite Chains: Chemical Bonding Evolution in AX3 Polyhalides under Pressure

Polyhalides are molecular systems that defy conventional views of chemical bonding, being infinite linear halide chains the most challenging systems. By studying CsI3 under compression, we show how I3‒ polyanions, with electron-rich multicenter bonds, polymerize giving rise to infinite linear iodine chains, I∞, and demonstrate that these chains, and, by extension, infinite linear halide chains, feature electron-deficient multicenter bonds. This result is in sharp contrast with previous assumptions that considered this type of bond to be impossible in valence electron-rich elements as halogens. Importantly, the process of formation of electron-deficient multicenter bonds in CsI3 at high pressure agrees with the recently proposed unified theory of multicenter bonding.