![]() ![]() The concept was further developed in the Polyhedral Skeletal Electron Pair theory (PSEPT) also known as Wade–Mingos rules based on a molecular orbital treatment of the bonding. The bonding in boron clusters of boranes and hydroborates was explained by Lipscomb using the concept of a three-electron-two-center bond. Inorganic hydroborates are salts or coordination compounds where one of the ligands is the hydroborate. They are also sometimes referred to as “boranes” this term is, however, used for neutral molecules B xH y according to IUPAC. Hydroborates are anions containing hydrogen bonded to boron. This review on the crystal chemistry of hydroborates is a contribution that should serve as a roadmap for materials engineers to design new materials, synthetic chemists in their search for promising compounds to be prepared, and materials scientists in understanding the properties of novel materials. All non-molecular hydroborate crystal structures can be derived by simple deformation of the close-packed anionic lattices, i.e., cubic close packing ( ccp) and hexagonal close packing ( hcp), or body-centered cubic ( bcc), by filling tetrahedral or octahedral sites. ![]() For bigger cations, the predictive power of the first Pauling rule is very poor. The Pauling rules for ionic crystals apply only to smaller cations with the observed coordination number within 2–4. The latter is determined from anion–anion coordination with the help of topology analysis using the program TOPOS. The coordination polyhedra around the cations, including complex cations, and the hydroborate anions are determined and constitute the basis of the structural systematics underlying hydroborates chemistry in various variants of anionic packing. Hydroborates containing anions other than hydroborate or neutral molecules such as NH 3 are not discussed. Carbaborane anions and partly halogenated hydroborates are included. Only the compounds with hydroborate as the only type of anion are reviewed, although including compounds gathering more than one different hydroborate (mixed anion). The crystal structures of inorganic hydroborates (salts and coordination compounds with anions containing hydrogen bonded to boron) except for the simplest anion, borohydride BH 4 −, are analyzed regarding their structural prototypes found in the inorganic databases such as Pearson’s Crystal Data.
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