RESUMEN
A combined study using neutron diffraction, inelastic neutron scattering, and first-principles calculations describe cobalt with a very low formal oxidation state of (-I) in a slightly distorted tetrahedral Co(-I)H4-complex in BaH2Mg5[Co(-I)H4]2 and in the structurally related RbH2Mg5[Co(-I)H4 Ni(0)H4]. This indicates that the electron "back donating" effect via the polarizable hydride ions to the counterions in the solid state hydrides, can be compared to more conventional "back bonding" able to reduce the oxidation state down to -I. The hydrides were synthesized by hot sintering of transition metal powders with corresponding binary alkali- and alkaline earth hydrides. In the similarly synthesized SrH2Mg2[Co(I)H5], cobalt is formally + I-valent, showing a high sensitivity to differences in the counterion framework, which can also influence electrical properties.
RESUMEN
The hydrogenous Zintl-phase Cs10H[Ga3H8]3 containing propane-like polyanions [Ga3H(D)8](3-) was successfully synthesized by direct hydrogenation of a 1:1 Cs/Ga metal mixture and characterized by powder X-ray and neutron diffraction. The charge of the polyanions is balanced by two different species of cations, hydrogen-centered octahedra [H(D)Cs6](5+) and isolated Cs(+). The structure crystallizes in the hexagonal space group P63/mcm (193) with the cell parameters a = 11.1108(3) Å, c = 18.2149(5) Å, Z = 2.
RESUMEN
The stability of a negatively charged polyethylene-structured [GaH2]n(n-) cluster ion was investigated by varying the K(+)/Rb(+) ratio in ((K(x)Rb(1-x))n[GaH2]n (0 ≤ x ≤ 1). Neutron, X-ray, and IR spectroscopies were used to characterize the new phases. Between the limiting compositions Kn[GaH2]n and Rbn[GaH2]n, the [GaH2]n(n-) chains remained almost identical, indicating a stable specie. For rubidium-rich samples up to a potassium content corresponding to (K0.5Rb0.5)n[GaH2]n, two phases coexist in the samples, RbGaH2 and (K0.5Rb0.5)n[GaH2]n, with a ratio mirroring the relative alkali-ion content. The two phases have a different alignment of the [GaH2]n(n-) chains. For potassium-rich samples beyond (K0.5Rb0.5)n[GaH2]n, the samples were single-phased. The unit cell volume of the new (K0.5Rb0.5)n[GaH2]n structure type shrinks according to Vegard's law as smaller K(+) ions substitute for larger Rb(+) ions. The [GaH2]n(n-) chains remained, however, almost identical. IR spectra from the different phases were very similar, exhibited stretching, scissoring, and rocking modes similar to those in ordinary polyethylene, but shifted to lower frequencies, reflecting weaker Ga-H bonds. The existence of stable Kn[GaH2]n and Rbn[GaH2]n, would help to dehydrogenate KGaH4 and RbGaH4 upon heating. If this could be transferred to lighter alanates and borohydrides, it could be possible to develop more functional hydrogen-storage systems.
RESUMEN
The first examples of a new class of gallium hydride clusters with direct Ga-Ga bonds and common hydrocarbon structures are reported. Neutron powder diffraction was used to find a Ga[GaH(3)](4)(5-) cluster ion with a neopentane structure in a novel cubic structure type of Rb(8)Ga(5)H(15). Another cluster ion with a polyethylene structure, [GaH(2)](n)(n-), was found in a second novel (RbGaH(2))(n) hydride. These hydrocarbon-like clusters in gallium hydride materials have significant implications for the discovery of hydrides for hydrogen storage as well as for interesting electronic properties.
