Anion structure, alanates

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]. 

Insoluble MH hydrides react very slowly, they must be used in large excess and rearrangements often occur. In some cases, however, the addition of the hydride ion H- was observed, and the formation of anionic complexes of poor stability, CpjLnH-, was mentioned [14]. Nevertheless, an yttrium hydride could be structurally characterised [15]. Owing to the high electropositive character of the lanthanide atoms, the use of borohydride (or aluminohydrides) reagents leads to stable borohydrides (or to bimetallic Ln-Al complexes) [1]. The spontaneous elimination of the borane (or alane), which is commonly observed for the late transition metals, does not occur. 

An interesting new area of manganese(I) chemistry is opened up by the recent report of the first transition metal compound of the tetrahydroalane(III) anion [Mn(AlH4)(dmpe)2] (where dmpe = l,2-bis(dimethylphosphino)ethane. This yellow, diamagnetic compound was prepared from [MnBr2(dmpe)2] and excess UAIH4 in toluene. It appears to be monomeric in solution, with a bidentate AlH moiety, but an X-ray structure determination of the solid shows that two alane moieties bridge to form the dimer (2). 

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