Transition metals interstitial hydrides

There are basically three types of hydrides ionic, covalent and transitional metal-interstitial hydrides. 

The interstitial carbides These are formed by the transition metals (e.g. titanium, iron) and have the general formula M, C. They are often non-stoichiometric—the carbon atoms can occupy some or all of the small spaces between the larger metal atoms, the arrangement of which remains essentially the same as in the pure metal (cf. the interstitial hydrides). 

The transition metal structures consist of close-packed (p. 26) arrays of relatively large atoms. Between these atoms, in the holes , small atoms, notably those of hydrogen, nitrogen and carbon, can be inserted, without very much distortion of the original metal structure. to give interstitial compounds (for example the hydrides, p. 113). 

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. 

The interstitial hydrides of transition metals differ from the salt-like hydrides of the alkali and alkaline-earth metals MH and MH2, as can be seen from their densities. While the latter have higher densities than the metals, the transition metal hydrides have expanded metal lattices. Furthermore, the transition metal hydrides exhibit metallic luster and are semiconducting. Alkali metal hydrides have NaCl structure MgH2 has rutile structure. 

The examples in Table III, show that the hydrogen atoms occupy tetrahedral holes at the beginning of the transition series. As we move along the transition series, we observe the interstitial hydride shift toward octahedral holes and the hydrides of the heavier elements become progressively unstable. Palladium is exceptional since it is the only heavy element of group VIII that gives a simple hydride. Hydride formation is accompanied in most cases by a change in metallic lattice type and in all cases by a considerable increase in metal-metal distances. 

The shift from tetrahedral to octahedral interstitial position is accompanied by a considerable increase in the hydride atom s apparent dimension, which can indicate that the late transition metals are more electron rich and more prone to give up a partial negative charge in favor of the electronegative interstitial atom. 

Finally, the heavier group VIII transition metals reluctance to form stable interstitial hydrides could be related to the higher values of the metal-metal interactions (I), as discussed in the following section. 

Hydrogen forms three types of binary hydrides. Active metals give ionic hydrides, such as LiH and CaFF nonmetals give covalent hydrides, such as NH3, H2O, and HF and transition metals give metallic, or interstitial, hydrides, such as PdH,.. Interstitial hydrides are often nonstoichiometric compounds. 

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... 

In a metal, certainly the transition metals, the electrons are more or less free to move in conduction bands. This fact is responsible for the high electrical conductivity of metals. When hydrogen atoms are present in the holes between the atoms, the movement of the electrons is somewhat impaired. As a result, the metal hydrides of this class are poorer conductors than the pure metals. The presence of hydrogen atoms makes the metal atoms less mobile and more restricted to particular lattice sites. Accordingly, the interstitial metal hydrides are more brittle than the parent metal. Also, the inclusion of the hydrogen atoms causes a small degree of lattice expansion so that the interstitial hydrides are less dense than the parent metal alone. 

The transition metal hydrides exhibit such wide variations from stoichiometric compositions that they have often been considered interstitial solid solutions of hydrogen in the metal. This implies that the metal lattice has the same structure in the hydride phase as in the pure metal. That this is not the case can be seen in Table I, where of 28 hydrides formed by direct reaction of metal and hydrogen, only three (Ce, Ac, Pd) do not change structure on hydride formation. Even in these three cases, there is a large discontinuous increase in lattice parameter. The change in structure on addition of hydrogen plus the high heats of formation (20 to 50 kcal. per mole) (27) indicates that the transition metal hydrides should be considered definite chemical compounds rather than interstitial solid solutions,... 

Hydride ligands are the most versatile of all organometallic ligands. Hydrides can coordinate to a single metal (terminal coordination), bridge two p-R, 4) or three (/U.3-H, 5) metal atoms, or occupy an interstitial position inside a metal polyhedron (6). Hydrides bound to transition metal clusters most commonly bridge two metal atoms. Interconversion... 

Several transition metals such as V, Nb, Ta, and Pd can form stable bulk hydrides, so-called interstitial hydrides the bonding in the hydride phase is not ionic but mostly metallic in character, and the hydrogen to metal ratio is not necessarily stoichiometric. Especially, nanoparticles of noble metals such as Pd are relatively easy to prepare by various methods, such as vapor phase deposition on substrates, reductions of salts in solution (electrochemically or electroless), and the inverse micelle templated growth. They are not easily oxidized, and, in recent years, several methods have been developed to precisely control the size of the particles or clusters. Furthermore, growth in solution in the presence of surfactants and stabilizers allows control over the shape of the final particles [35, 36, 42]. 

Although hydrogen can react with transition metals to form compounds such as UH3 and FeH6, most of the interstitial hydrides have variable compositions (often called nonstoichiometric compositions) with formulas such as LaH2.76 and VHo.56- The compositions of the nonstoichiometric hydrides vary with the length of exposure of the metal to hydrogen gas. 

---