Neutron diffraction metal hydrides studied

Hydrides. According to X-ray and neutron diffraction and metallographic studies of the Nb-H system,524 the H may be considered a lattice gas with phase transitions. In the a-, a -, j6-, and -phases of the system, H occupies tetrahedral interlattice positions. Whereas direct reaction between niobium metal and hydrogen occurs only after repeated activation of the metal by hydrogen absorption at ca. 7 atm and 350 °C, NbH2 is formed at temperatures as low as 22 °C in mixtures of LaNi5H6 7 and Nb.525 The extraordinary catalytic effect of the lanthanum-nickel complex is attributed to the presence of surface-absorbed atomic hydrogen species which are able to diffuse into the niobium lattice. There has been a review of the T a-H system.526... 

Neutron diffraction studies have shown that in both systems Pd-H (17) and Ni-H (18) the hydrogen atoms during the process of hydride phase formation occupy octahedral positions inside the metal lattice. It is a process of ordering of the dissolved hydrogen in the a-solid solution leading to a hydride precipitation. In common with all other transition metal hydrides these also are of nonstoichiometric composition. As the respective atomic ratios of the components amount to approximately H/Me = 0.6, the hydrogen atoms thus occupy only some of the crystallographic positions available to them. 

Neutron Diffraction Studies of Tetrahedral Cluster Transition Metal Hydride Complexes HFeCo3(CO)9-(P(OCH3)3)3 and H3Ni4(C5H5)4... 

In 1970, Frenz and Ibers reported the results of four neutron diffraction experiments in their structural review ). Since that time, over four dozen metal hydride complexes have been studied by this technique (see following tables). Based on these and supplemental X-ray results, some generalizations can be tentatively formulated ... 

Neutron and X-Ray Diffraction Studies of Nonstoichiometric Metal Hydrides... 

A systematic diffraction study was made with both neutrons and x-rays of metal- hydride systems in the composition range of 2 to 66.5 atomic % hydrogen of hafnium, titanium, and zirconium, and a nuclear null-matrix consisting of 62 atomic % titanium and 38 atomic % zirconium, with emphasis on the metal-rich regions. A nuclear null-matrix as defined here consists of two or more types of nuclei in which some of the nuclei scatter thermal neutrons 180° out of phase with others, such that the resultant structure factor is zero. 

X-ray and neutron diffraction studies show that in these hydrides the H ion has a crystallographic radius between those of F and Cl". Thus the electrostatic lattice energies of the hydride and the fluoride and chloride of a given metal will be similar. These facts and a consideration of the Bom-Haber cycles lead us to conclude that only the most electropositive metals can form ionic hydrides, since in these cases relatively little energy is required to form the metal ion. 

The hydride decomposes at somewhat higher temperatures to give extremely reactive, finely divided metal. A study of the isostructural deuteride by X-ray and neutron diffraction shows that the deuterium atoms lie in a distorted tetrahedron equidistant from four uranium atoms no U—U bonds appear to be present, and the U—D distance is 2.32 A. The stoichiometric hydride UH3 can be obtained, but the stability of the product with a slight deficiency of hydrogen is greater. 

The metal carbonyls of group 9 undergo reduction to the tetracarbonytmetalates(-I) (see equation 45 for Co2(CO)g). Acidification of an aqueous solution of Na[Co(CO)4] followed by extraction into an organic solvent produces the hydride CoH(CO)4 (see equation 46), which is a sufficiently strong acid to be converted to the trialkylammonium derivative by tertiary amines R3N (R = Me, Et) (equation 47). The C3 symmetry of CoFI(CO)4 in the gas phase is maintained in the trialkylammonium derivatives as shown by X-ray and neutron-diffraction studies. ... 

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