Planar Pentacoordinate Halogens

Planar hypercoordinate motifs represent an intriguing frontier in chemistry, challenging traditional bonding norms. As electronegativity of the central atom increases, achieving planar hypercoordination becomes more difficult due to restricted delocalization, making the design of planar hypercoordinate halogens particularly puzzling. Here, we conduct an extensive computational survey of Li_nX_n+1 ^- (n=4, 5, 6; X=F, Cl, Br, I) clusters, revealing a starlike D_5h-symmetry global minimum in Li₅X₆ ^- (X=F, Cl, Br) with a planar pentacoordinate halogen (ppX), where X^- is located at the center of Li₅X₅ crown. The clusters are stabilized predominantly through electrostatic interactions between X^- and Li₅X₅, complemented by weak covalent bonding from dative interaction. Due to the weak orbital overlap, ppX clusters exhibit localized diatropic ring currents around X and Li.