Compounds, chemical, formation interstitial

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,... 

Metallic hydrides are usually nonstoichiometric compounds, as expected from their relatively low heats of formation and the mobility of hydrogen. They are ordinarily described, chemically, in terms of any of three models in which hydrogen is considered a small interstitial atom, a proton, or a hydride anion. These models are discussed critically with particular reference to the group V metal hydrides. The interstitial atom model is shown to be useful crystallographically, the protonic model is questioned, and the hydridic model is shown to be the most useful at present. The effect of hydrogen content on the lattice parameter of VHn and the structural and magnetic properties of several hydrides are discussed in terms of these models. 

Elemental analysis generally poses no problems because of the limited stability of the compounds and the formation of elemental gold in decomposition and combustion, which does not form carbides, nitrides, or other interstitial phases. Atomic absorption and inductively coupled plasma spectroscopy are presently the methods of choice for An estimation. Many organogold compounds are sufficiently volatile to allow registration of good mass spectra by gas-phase electron impact. Field desorption, fast-atom bombardment, and chemical ionization mass spectrometry have also been successfully applied. 

Since platinum, rhodium, and ruthenium catalysts operate with similar activation energies, their differences in catalytic activity can be directly traced to differences in the A factor, which may be related to the % d-char-acter of the metal bond in the three metals above. Since the % d-character is 50, 50, and 44 for ruthenium, rhodium, and platinum, respectively (S), it is seen that this sequence is similar to that of the catalytic activity. During catalysis, the palladium surface becomes a chemical compound represented by various stages of interstitial hydride formation, whose d-charac-ter is essentially different from that of the metal. Therefore, the position of palladium in the % d-character sequence is not directly comparable to that of palladium in the catalytic activity sequence. 

In these compounds, non-metal atoms penetrate into the crystal lattice of tremsition metal with the result the crystal lattice gets slightly distorted. These compounds resemble the parent transition metal in chemical properties but differ in some physical properties like density, hardness and conductivity etc. For example, steel and cast iron are hard because of the formation of interstitial compoimds with carbon. 

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