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metal hydrides; high pressure synthesis; crystal structure; neutron diffraction; magnetic properties
Abstract:
Previously unknown ternary metal hydrides A(x)OsH(z) and A(x)RuH(z) (A=alkali metal) were synthesized by the reaction of alkali metal hydrides with osmium or ruthenium under a hydrogen pressure up to 6000 bar. The crystal structures of the hydrides were determined using a combination of X-ray and neutron diffraction experiments with deuterated compounds on powdered samples. Na3OsH7 and Na3RuH7 crystallize at room temperature in a tetragonal structure type (space group: P4(2)/mnm), which is characterized by isolated [OsH7]- or [RuH7] anions. The coordination polyhedron formed by the seven hydrogen (deuterium) ligands can be described as a distorted pentagonal bipyramide. Magnetic susceptibility measurements on Na3OsH7 in the temperature range between 3.5 K and room temperature revealed a weak temperature independent paramagnetism. Quantum mechanical calculations confirm these facts and show in detail that the large value of the spin-orbit coupling constant is responsible for the magnetic behaviour of these compounds with the oxidation state +4 for the transition metal atoms. With the heavier alkali metals transition metal(IV) hydrides A(3- delta)OsH(7-delta) and A(3-delta)RuH(7-delta) could be synthesized. They crystallize with isotypic atomic arrangements related to the cryolite structure type. Highest hydrogen pressure during the reaction leads to the osmium(VI) hydrides Cs3OsH9 and Rb3OsH9. The atomic arrangement is characterized by [OsH8] polyhedra intercalated in a cubic HA(3) framework which corresponds to the ReO3 structure type. Within these polyhedra the hydrogen ligands occupy two 24-fold positions with a statistical distribution. (C) 2002 Elsevier Science BY All rights reserved.
