Alkaline earth metal alanates

In recent years, a considerable amount of research has been done on the preparation and characterization of alkaline earth metal alanates. A detailed procedure for the preparation of solvent-free magnesium alanate has been described by Fichtner and Fuhr [198]. Lately, Fichtner and Ahlrichs et al. [199], starting from FTIR data of Mg(AlH4)2 and its solvent adducts, performed a quantum chemical calculation... 

It should be noted that the thermal dissociation of alkaline earth metal alanates (Eqs. (6.26) and (6.27)) with formation of pentahydrides MAlEls follows a different pathway than that of alkali metal alanates (Eqs. (6.15), (6.19) and (6.20)), whose intermediates are hexahydrides of the type M 3A1H6, M = Li, Na, K. Knowledge of the dissociation pathways of alkali and alkaline earth metal alanates and their structures (see below) and the relative stability of their intermediates can be useful in the search for new hydrogen storage systems based on alanates. 

In contrast to above alanates, Mg(AlH4)2 of the alkaline-earth metal alanate decomposes without the formation of the intermediate phase MfAlHe). Fichtner et al. reported that the dehydrogenation of Mg(AlH4)2 proceeds according to the following reactions [53] ... 

In the first stage of alan method we carried out interaction of binary hydrides of alkaline, alkaline-earth metals and aluminum as etherate A1H3 nEt20 (or AIH3) in mole ratio MH2 A1H3 = 1 2 in the medium of ether with intensive mixture and operation mode with surplus of A1H3 ... 

The best route to Al—H bonds from the elements is the formation of tetrahydroaluminates, because these reagents are convenient to handle and may be used to prepare other alanes. The reaction between an alkali- or alkaline-earth metal and ac-... 

The state of the art of reductions with metal hydrides a decade ago was the subject of comprehensive reviews. A detailed survey of reductions of carbonyl compounds with alkali and alkaline earth metal hydrides, borane and derivatives, alane and derivatives, metal borohydrides, metal aluminohydrides, silanes, stannanes and transition metal hydrides was compiled. The properties, preparation and applications of each reagent were discussed together with methods for their determination, handling techniques... 

Trialkyl alanes also form association products with the alkyls of the alkali and alkaline earth metals, but these are very stable and some are saltlike complexes (cf. Section IV,C). An alkyl exchange such as... 

According to thermovolumetric measurements and IR and XRD evidence, the thermal dissociation of aU three alanates follows the same hydrogen evolution pattern [201-204] formation of the pentahydride (Eq. (6.26)), formation of the dihydride (Eq. (6.27)) and decomposition of the dihydride with formation of the corresponding alkaline earth metal-aluminum intermetallic compound (Eq. (6.28)). The pentahydrides can also be obtained by ball-milling M(A1H4)2 with dihydrides (Eq. (6.29)). 

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. 

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