Complex hydrides classification

This chapter summarizes important contributions on the AB5/AB and elemental hydrides. Examples of microalloying in LaNis- and FeTi-based intermetallic hydrides have been emphasized, and there has been some discussion on elemental hydrides. Classification of testing methods for durability of alloys/ intermetallics were presented, examples of intrinsic and extrinsic tests that include cycling and aging have been reported for AB5/AB and other alloys. More recent results on extrinsic tests on lightweight complex hydrides (imide-amide) that were cycled between Li2NH/LiNH2 phases were also reported. 

The hydrides of the later main-group elements present few problems of classification and are best discussed during the detailed treatment of the individual elements. Many of these hydrides are covalent, molecular species, though association via H bonding sometimes occurs, as already noted (p. 53). Catenation flourishes in Group 14 and the complexities of the boron hydrides merit special attention (p. 151). The hydrides of aluminium, gallium, zinc (and beryllium) tend to be more extensively associated via M-H-M bonds, but their characterization and detailed structural elucidation has proved extremely difficult. 

Figure 5 A classification of the hydrides. The starred elements are the transition elements for which complex molecules or ions containing M-H bonds are known. (Ref 84. Reproduced hy permission of Wiley Inc.)...

These complexes barely deserve a separate classification in that their chemistry is very similar to that of the simple carbonyl hydrides. All are diamagnetic and their formulation obeys the rare gas rule. Only rarely does the jr-cyclopentadienyl group play a distinctive role in the chemistry of these complexes, and for the majority of the chemistry it may be regarded as an inert ir-bonding ligand. 

Complexes with H-H distances between 1.0 and 1.5 A are often referred to as elongated or stretched dihydrogen complexes [57]. The properties of these complexes make it difficult to justify their classification as either trae dihydrogen complexes (dH-H<10A) or ds-dihydride complexes (dH-H>l-5A). Other authors further categorize these complexes as true elongated dihydrogen complexes (dn-H < 10-1.3 A) and compressed bis-hydrides (dn-H < 13-1.5 A) [46]. 

Table 28 A classification and some examples of transition metal hydride complexes...

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