Hydride complex, interstitial metal

Attempts to classify carbides according to structure or bond type meet the same difficulties as were encountered with hydrides (p. 64) and borides (p. 145) and for the same reasons. The general trends in properties of the three groups of compounds are, however, broadly similar, being most polar (ionic) for the electropositive metals, most covalent (molecular) for the electronegative non-metals and somewhat complex (interstitial) for the elements in the centre of the d block. There are also several elements with poorly characterized, unstable, or non-existent carbides, namely the later transition elements (Groups 11 and 12), the platinum metals, and the post transition-metal elements in Group 13. 

In contrast to the interstitial hydrides, where the metal lattice hosts the hydrogen atoms on interstitial sites, the desorption of the hydrogen from the complex hydride leads to a complete decomposition of the complex hydride and a mixture of at least two phases is formed. For alkali metal tetrahydroborates and tetrahydroaluminates, the decomposition reaction is described according to the following equation ... 

Hydride complexes containing more than one T metal atom are less common but of considerable interest because of their possible links to the so-called interstitial hydrides. Those reported so far aU display T—H—T bridges and some also T—T bonds. 

Figure 5.32 Comparison of metal hydrides with complex hydrides [50]. Metal hydrides consist of an almost unchanged metal lattice with the interstitial sites filled with hydrogen. The additional electrons from hydrogen are added at the Fermi level of the metallic electron density of states and an additional hydrogen-induced band several eV below the Fermi level accommodates...

The crystal structure and stoichiometry of these materials is determined from two contributions, geometric and electronic. The geometric factor is an empirical one (8) simple interstitial carbides, nitrides, borides, and hydrides are formed for small ratios of nonmetal to metal radii, eg, rx / rM < 0.59. When this ratio is larger than 0.59, as in the Group 7—10 metals, the structure becomes more complex to compensate for the loss of metal—metal interactions. Although there are minor exceptions, the H gg rule provides a useful basis for predicting structure. 

While DFT may or may not be more accurate than MP2 for absolute shielding calculations is debatable, the strength of the DFT method in calculations of shieldings is in the ability of DFT to provide a consistent picture over a wide range of chemical systems, since calculations can be done at a very modest computational cost compared to MP2. Among the successes of the method is in ligand chemical shifts in transition metal complexes. For example, 13C, 170,31P and H chemical shifts for oxo (12,14,15), carbonyl (16-19), interstitial carbide (20), phosphine (21,22), hydride (23), and other ligands have been successfully reproduced to within tens of ppm in... 

Some solid-state metal hydrides are commercially (and in some cases potentially) very important because they are a safe and efficient way to store highly flammable hydrogen gas (for example, in nickel-metal hydride (NiMH) batteries). However, from a structural and theoretical point of view many aspects of metal-hydrogen bonding are still not well understood, and it is hoped that the accurate analysis of H positions in the various interstitial sites of the previously described covalent, molecular metal hydride cluster complexes will serve as models for H atoms in binary or more complex solid state hydride systems. For example, we can speculate that the octahedral cavities are more spacious in which H atoms can rattle around , while tetrahedral sites have less space and may even have to experience some expansion to accommodate a H atom. 

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