Hydrides AH

Alkaline-Earth Metal Hydrides. Table 2 gives thermochemical data of alkaline-earth metal hydrides. AH form orthorhombic crystals. 

The bonding radius of A in a diatomic hydride AH at first decreases across a period and then increases because there is a competition between two opposing effects ... 

The success of calculations including correlation, described above, should not obscure the fact that such calculations are (i) relatively expensive, and (ii) not easy to apply to much larger molecules. Therefore, attempts to compute the correlation energy semi-empirically that can be applied to larger molecules are of considerable interest. Clementi has for many years advocated such an approach, and in a recent series of papers, Lie and dementi123 have demonstrated one such method for first-row hydrides (AH) and diatomics (A2). 

Figure 19.6 Correlation between enthalpy change of the perovskite-type hydrides, AH, and enthalpy change of the binary hydrides, AH2, in the dehydrogenation reaction.

Diatomic Hydrides—AH. A listing of electronic configurations and term types for the ground and first excited states of the higher diatomic hydrides as well as the first- and second-row diatomic hydrides is given in Herzberg [2]. 

The atomic density of hydrogen in many metal hydrides is greater than that in liquid H2 or in H20. Metal hydrides are efficient moderators (Fig. 1) and neutron shielding materials, and help to minimize the core shield volume. Metal-clad yttrium hydride moderators capable of operation at 1000°C in air, uranium-zirconium hydride rods as a combination fuel-moderator element are examples, and metal-clad zirconium hydride units as moderator elements for operation up to 600°C° °. The hydrogen atom density in hydrides, Ah, the number of hydrogen atoms per cubic centimeter of hydride X 10 , is calculated from the hydrogen-to-metal atom ratio, H/M, the density of the hydride p, and the molecular weight W by ... 

Tables of electronic wavefunctions have been compiled for the diatomic hydrides AH, where A denotes the elements Li through F, and Na through Cl.195 X-Ray diffraction patterns at high pressure show that LiH retains the low-pressure NaCl structure up to 12.0 GPa (120 kbar).196 The preparation of the first stable complex metal hydride of copper, lithium dihydro-cuprate(i), is reported from the reduction of LiCuMe2 by lithium aluminium hydride in ether at low temperatures. The solid compound, LiCuH2, is solvated by ether and stable under ambient conditions for several days.197...

The second-row diatomic hydrides AH, A = Li to F, with data given in Table 2, exhibit all three types of surfaces. This is made clear in Figure 2, which displays the arms of the interatomic surfaces for the hydrides relative to a fixed position of the proton. The change in the direction of charge transfer, initially from A to H up to boron and from H to A beyond carbon (which has close to zero charge transfer) is evident in the advance of the surface towards the proton - the advance of the bond critical point - the... 

The hydrides AH of all the alkali metals are known they all form stable, colourless crystals of relatively high melting point and all have the sodium chloride structure in which the hydrogen occurs as the negative ion H" of radius 1 54 A. This radius is intermediate between... 

Figure 4.19 Data from Bader, R.E.W, Keaveny, L, and Cade, RE. (1967) Molecular charge distributions and chemical binding II. First-row diatomic hydrides, AH , J. Chem. Phys. 47, 3381.

Fig. 1.35. Representations of the atoms in the second-row hydrides AH . In the hydridic members LiH, BeH2, and BH3, the A atom consists primarily of a core of decreasing radius, dressed with some residual valence density. The form and properties of the atoms undergo a marked change at methane, a nonpolar molecule no core is visible on the C atom, and the H atoms, considerably reduced in size and population, now lie on the convex side of the interatomic surface. The increasing polarity of the remaining members is reflected in the decreasing size of the H atom and the increasing convexity of its interatomic surface. (Reproduced from Angew. Chem. Int. Ed. Engl. 33 620 (1994) by permission of Wiley-VCH.)...

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