Hydrides octahedral site

The existence of H atoms of a higher coordination number (four, five or six) is an intriguing problem. It was not until very recently that examples of such species were unequivocally established in molecular complexes246,247). In the solid state literature, however, high-coordination H atoms are well-known entities there are many examples of binary metal hydrides in which H atoms are known to occupy tetrahedral or octahedral sites in a metal lattice8). 

The molecule with X = S is simply and quantitatively made by passing H2S though a refluxing octane solution of Os3(CO)12 (46). The X-ray and neutron diffraction structures of Os3H2(S)(CO)9 (206) and the X-ray structure of the anion [Os3H(S)(CO)9] (207) are known. In the neutral compound the hydride ligands occupy octahedral sites at osmium so that Os—H—Os... 

Figure 1. Calculated properties of hydride of BCC metal (atomic radius unity) as a function of radius R of hydrogen in octahedral sites (1/2,1/2, 0) and (0,0,1/2)...

The relation of H content, n, to lattice parameter for VH is interpreted readily by the hydridic model as indicated in Figure 5, which represents a portion of a hypothetical unit cell in which H-, of radius 1.22 A., is located in an octahedral site in a BCC V+5 cell. The radius of V+5 is 0.48 A., both ionic radii being corrected for fourfold coordination (12). The V-H distance is, of course, the same as that given by the atomic model shown on the right, where the metal and hydrogen radii are, respectively, 0.93 and 0.56 A. (see also Figure 6). 

On hydrogenation, hydrogen atoms vhll occupy specific interstitial sites. The interstitial sites in three major crystal structures of hydrides are shown in Figure 4.6. Only octahedral (O) and tetrahedral (T) sites are shown because they are the only ones occupied by hydrogen atoms. However, some distinction should be made between the different structures. In the fee lattice, the Tand O sites are, respectively, enclosed in regular tetrahedral and octahedral formed by metal atoms. At low or medium H concentration, the preferred interstitial sites are octahedral. In the hep lattice, the tetrahedral or octahedral sites become distorted as the ratio of lattice parameters eja deviates from the ideal value of 1.633. Tetrahedral sites are favored at low H concentration in hep metals. In fee and hep lattices, for each metal atom, there is one octahedral site and 2 tetrahedral sites available for hydrogen after considering the Westlake criterion. 

When nitrogen impurities are introduced into the tantalum (they reside on octahedral sites) the hydride, TaNo.oo6Ho.oo3, behaves differently. Firstly, they retain their tetrahedral frequencies, discounting suggestions that the hydrogen atoms have moved to octahedral sites. Moreover, this spectrum remains unchanged down to the lowest temperatures, 1.5 K, showing that the solid solution phase has not precipitated [59]. 

In addition to this electronic interaction between finely divided ruthenium metal particles and cerium hydride, a hydrogen spillover was also proposed to explain the high activity for ammonia synthesis of these catalysts. Since hydrogen in the octahedral sites of CeH2+.t is desorbed at 420°C, atomic hydrogen is certainly present at the temperature of ammonia synthesis (450-550°C). Therefore spillover of atomic hydrogen at the hydride/ transition metal interface could occur. 

An interstitial H atom located in a tetrahedral environment was first reported in 1982, in the tetracapped octahedral cluster [HOsioC(CO)24] . Evidence came from X-ray studies which showed complete coverage of the metal core by carbonyl ligands very similar to that of [OsioC(CO)24] from which it was prepared. It was therefore assumed that the hydride was in an interstitial site, and because the octahedral cavity was already occupied by a carbido-atom it seemed reasonable to assume that the hydride was sited within a tetrahedral cap. The Os satellite pattern associated with the hydride signal in the H NMR spectrum was entirely as would be expected for a tetrahedrally coordinated hydride, which helped to confirm this proposal. 

Although less fully documented than osmium cluster chemistry, rhenium cluster chemistry has been subjected to many structural studies, including those on approximately 20 neutral or anionic carbonyls, particularly carbonyl hydrides [Rev(CO). H ] of nuclearities x = 2 to 6 (Fig. 7). In addition, some ten or more rhenium carbonyl carbides [Rev(CO)vH C] have been shown to contain a core carbon atom, usually occupying a central octahedral site. These systems offer scope not only to explore for rhenium the trends we have already shown for osmium, but also to study the effect on metal-metal distances (and so enthalpies) of such core carbon atoms, which formally donate all four of their valence shell electrons to the cluster bonding. To our knowledge only one rhenium carbonyl cluster compound, Re2(CO)io, has been subjected to calorimetric study to determine its enthalpy of formation. ... 

What criterion deddes whether an element is able to contribute as an interstitial atom to the stabilization of an electron defident duster Obviously, the M6 octahedron bears some relation to a microscopically small piece of metal. The vibrational frequent of the H atom in the [NbgH] unit of Nb(I,iH is nearly identical to the frequency of H in the metallic hydride NbH. [92] Since the bonding in the duster and that in the extended metal lattice are so similar, the obvious question to ask is which elements form stable compounds with the bulk metal that represents the duster atoms. The answer to this question yields a qualitative explanation for the fact that the electron defident Nb unit incorporates H, while the Mog octahedron does not. Zr forms numerous intermetallic compounds with Be, Al, and other d metals and, obviously, does not loose this ability when only six Zr atoms are joined. Inspection of the experimental data for the relevant binary systems or the use of Miedema s concept for the stability of intermetallic systems [93, 94] proves helpful in the search for possible interstitial atoms, and naturally limiting the search to atoms of appropriate size to fit into the octahedral site. Of course, if the intermetallic compounds are very stable they could also compete with the duster compound formation. 

Many of these compounds (MHn) show large deviations from ideal stoichiometry (n = 1,2, 3) and can exist as multiphase systems. The lattice structure is that of a typical metal with atoms of hydrogen on the interstitial sites for this reason they are also called interstitial hydrides. This type of structure has the limiting compositiOTis MH, MH2, and MH3 the hydrogen atoms fit into octahedral or tetrahedral holes in the metal lattice, or a combination of the two types. In fee and hep structures, we find 1 octahedral site (r = 0.414) and 2 tetrahedral sites (r = 0.255), and in bcc (body-centered-cubic) structures we find 3 octahedral site (r = 0.155) and 6 tetrahedral sites (r = 0.291). 

Polymeric hydrides (white solids) are formed by Be and Al. In BeH2 (Fig. 10.15), each Be centre is tetrahedral, giving a chain structure in which multi-centre bonding of the type described for B2H5 is present The structure of AIH3 consists of an infinite lattice, in which each Al(lll) centre is in an AlHg-octahedral site H atoms bridge pairs of Al centres. 

The relative increase in intensity of the 1.6 eV peak at higher Ep is therefore evidence of a hydride phase increasing in H concentration with depth. The system is disordered, consistent with random occupation d tetrahedral sites, and a CeH (x < 2) stoichiometry. Heating to 550°C leads to partial H desorption, but at lower temperatures a different hydride phase may become established with possible octahedral sites occupation near the surface. 

The rare earths absorb hydrogen readily and form solid solutions and/or hydrides exothermally at temperatures of several hundred C. Their phase diagrams consist, in general, of three basic parts (a) the metallic solid solution, or a-phase, with the H atoms inserted in the tetrahedral interstices of the host-metal lattice (b) the equally metallic dihydride 3-phase, where the two H atoms occupy ideally the two available tetrahedral sites this phase crystallizes in the fee fluorite system (c) the insulating trihydride, or y-phase, which possesses an hep unit cell with both tetrahedral sites and the one octahedral site filled up. A schematic view is given in fig. 1. Exceptions are the divalent lanthanides Eu and Yb, whose dihydrides are already insulators and exhibit an orthorhombic structure, and Sc whose very small unit cell does not normally accept more than two H atoms. 

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