The bonding in hexagonal BaPtB and CeCoB type ternary metal borides.

Tight-binding calculations with an extended Hückel Hamiltonian were performed on BaPtB and LuOsB. Hypothetical linear metal boride chains present in these materials are analyzed with a three-dimensional model that contains a trigonal bipyramidal TB (T = transition metal) building unit for the compounds. The geometrical structure for the TB trigonal bipyramids depends on the number of electrons. For systems that have greater than 36 electrons in its trigonal bipyramidal building unit, a structural distortion is expected. Electron back donation from the electron-rich M fragment to the empty e′ set on B creates boron-boron interaction along the z-axis. Boron-boron pairing then participates as an electron sink and causes a trigonal distortion of the platinum Kagome net. On the other hand, a system with <35 electrons should have an undistorted, CeCoB type structure. The electronic factors that create the breathing motion are discussed and analyzed with the aid of molecular and solid-state models. The metal-metal bonding associated with the structural properties also has been examined. [ABSTRACT FROM AUTHOR]