Complex hydrides metal alanates

The crystal structures of all complex aluminum hydrides are built up by [AlH4] tetrahedra or [AlHg] octahedral units. These building units can be either isolated, as for example in NaAlH4, or they can form more complex structures such as chainlike structures, as for CaAlHs. The decomposition of alkaline earth aluminum hydrides proceeds via hydrides to intermetallic compounds whereas alkali metal alanates decompose via an intermediate hexahydride structure to the corresponding hydride. Table 5.2 summarizes the physical data of selected complex aluminum hydrides. 

Solid-state nuclear magnetic resonance (NMR) has been extensively used to assess structural properties, electronic parameters and diffusion behavior of the hydride phases of numerous metals and alloys using mostly transient NMR techniques or low-resolution spectroscopy [3]. The NMR relaxation times are extremely useful to assess various diffusion processes over very wide ranges of hydrogen mobility in crystalline and amorphous phases [3]. In addition, several borohydrides [4-6] and alanates [7-11] have also been characterized by these conventional solid-state NMR methods over the years where most attention was on rotation dynamics of the BHT, A1H4, and AlHe anions detection of order-disorder phase transitions or thermal decomposition. There has been little indication of fast long-range diffusion behavior in any complex hydride studied by NMR to date [4-11]. 

The metal hydrides exhibit different reducing properties. The complex metal hydrides (LiAlH4, NaBH4) are nucleophilic in character hydride is transported from the complex anion to the electron-deficient centers of the functional groups. Another group of metal hydrides (boranes, alanes) are strong Lewis acids they interact with centers that are relatively richer in electrons. The selectivity of the reduction can be improved and the scope of its application can be extended by the joint use of these two types (mixed hydrides). 

Common reducing agents are hydrogen in the presence of metallic or complex catalysts (e.g. Ni, Pd, Pt, Ru, Rh), hydrides (e.g. alanes, boranes, LIAIH, NaBHJ, reducing metals (e.g. Li, Na, Mg, Ca, Zn), and low-valent compounds of nitrogen (e.g. NjHj, NjHJ, phosphorus (e.g. triethyl phosphite, triphenyiphosphine), and sulfur (e.g. HO-CHj-SOjNa = SFS, sodium dithionite = Na S O. ... 

Ritchson, E., Nanotechnology and hydrogen storage in complex metal hydrides, available at http //engineering.purdue.edu/ChE/webpublications/ericjri/physics and chemistry of the alanate, March 2007. 

Closely related to, but distinct from, the anionic boron and aluminum hydrides are the neutral boron (borane, BH3) and aluminum (alane, A1H3) hydrides. These molecules also contain hydrogen that can be transferred as hydride. Borane and alane differ from the anionic hydrides in being electrophilic species by virtue of a vacant p orbital at the metal. Reduction by these molecules occurs by an intramolecular hydride transfer in a Lewis acid-base complex of the reactant and reductant. 

US DOE is investigating two types of hydrides for solid-state hydrogen storage complex metal hydrides and chemical hydrides. DOE s research on complex metal hydrides is focused on increasing the storage capacity of alanates, extending the durability and cycle lifetime, and improving uptake... 

Acknowledgments. Like editors of previous volumes of this series, I am indebted to many colleagues for contributions to this work. First there are Wolfgang Beck in Munich, Ekkehard Lindner in Tubingen, and John R. Shapley in Urbana, who assisted me in the selections and solicitations for three special collections transition metal complexes containing weakly bonded anions (Chapter 3), metalocyclic complexes (Chapter 4), and polynuclear transition metal complexes (Chapter 5), respectively. Each has contributed a preface for the chapter he helped to form, following the pattern set in Volume XII when Alan G. MacDiarmid invited E. C. Ashby and myself to form a chapter on metal hydrides. As a consequence of such efforts, more than two-thirds of this volume consists of invited preparations. 

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