Planar and Pyramidal Pentacoordinate Selenium Atoms.

A systematic computational exploration of the dianionic systems SeM5X5 2- (M = Li─Cs; X = F─I) was conducted to evaluate the viability of planar pentacoordinate selenium (ppSe). Of the twenty combinations considered, 10 adopt planar or pyramidal structures, with four corresponding to true global minima containing a ppSe center. Bonding analysis and magnetic response calculations show that the stabilization of ppSe arrangements in these dianions results mainly from geometric confinement and strong electrostatic interactions between selenium and the surrounding alkali metal and halogen atoms. Unlike planar hypercoordinate systems stabilized by delocalized bonding or aromaticity, the dianions studied here display highly localized electronic structures, expanding the scope of nonclassical bonding motifs in main-group chemistry.