Hydride structure

New multinary hydride structures (Li-Mg-B-N-H) were successfully synthesized and characterized using LiBfVLiNLL and MgH2. These hydrides show enhanced reaction kinetics at lower temperatures due to the nanocrystalline behavior of MgH2 having large surface area. Moreover, the catalytic addition drastically reduces the thermal decomposition temperatures of Li-Mg-B-N-H when compared to undoped complex hydrides. 

We now turn to the formation of some of these hydride structures. The majority of them are based on a fee array of metal atoms, as shown by the open circles in Figure 2. The dihydride structure comes from filling the tetrahedral interstice (large solid circles) in the lattice with hydrogens and gives the well known CaF2 or calcite structure. Similarly, if one fills the octahedral interstice (small solid circles), one gets the NaCl or rocksalt structure found in nickel hydride and palladium hydride, which we will discuss near the end of this chapter. 

Figure 2. Fee hydride structures. Large open circles represent metal atom positions, solid large circles represent hydrogen atom positions in dihydride structure, small solid circles represent hydrogen atom positions in NiH and PdH.

Summary of polyhedral polyboron-hydride structure-types, occording to stoichiometry and electroncounting relationships... 

The H-l NMR spectrum of the cyclometallated hydride (Structure XI) shows no hydride resonance at room temperature, but on cooling a broad resonance starts to appear, and at -62°C it shows resolved coupling to 31P (19 Hz) and 103Rh (55 Hz). We ascribe this temperature-dependent behavior to the occurrence of a rapid, reversible C-H/Rh-H fission such as shown in Scheme II. Alternatively there may be rapid, reversible oxidative additions, Structure XII Structure XI, as in Scheme I. 

Scheme I. Showing a possible sequence of reactions leading to the fluxional hydride (Structure XI)...

Scheme II. Possible processes to explain the fluxionality of the hydride (Structure XI) (A) concerted hydrogen shift mechanism (B) stepwise process involving a carbene intermediate.

An interesting aspect of the bridging hydride structure concerns the direction of the Jahn-Teller axis on nickel. With the hydride, there are five ligands around nickel and one of these are therefore forced to... 

Muetterties and Schunn (1966) suggest that certain boron hydride structures, e.g. 26 and 22 or 23 (C, symmetry, when X=Y), might be used as models for electrophilic substitution. In general, if 21 or 22 were the preferred C3 transition states or intermediates, the reaction would go with retention. Note that these are the analogs of the two-electron three-center species A3 discussed earlier by HMO theory. If, however, X, Y, and R were permuted, there would be several stereochemical possibilities, two of which are indicated in Table 7. To decide what kind of geometry is assumed in SE2 transition states, which are pentacoordinated and electron- and orbital-deficient, calculations on model species are needed to establish preferred geometries. 

Hydrogen homogeneity was controlled by metallographic examination. Metallography of hydride structure on radial-axial and radial-transverse sections shows a uniform hydride distribution with hydrides elongated in the longitudinal direction (Fig. 1). From the hydrided pressure tube material curved compact toughness (CTT) specimens were machined. Except for the thickness and the curvature of the tube, the in-plane dimensions of specimens were in proportion described for compact specimen in ASTM standard test method (E-399). 

C5(CH3)51M systems makes it unlikely that unwanted rearrangements, giving more stabilized dimeric (e.g., bridging hydride) structures, will occur. 

The Sm complex [254] is characterized by a formal twenty electron configuration. The Sm atom is coordinated by four hydrides. The heterometallic complex of Y and Re has an open triangular structure as shown in Fig. 6.26. The Re(2)-Y distance is 4.186 A and the other Re(l)-Y distance of 3.090 A. This suggests the presence of a hydride bridge for Re(l)-Y. Short Re-Re distance of 2.576 is also accounted for by the earlier explanation. The bridging hydride structure is also supported by NMR data [255]. 

Table 1 Complex transition metal hydride structure types and representatives...

Table 19 Homonuclear and polynuclear complex anions as found in various hydride structure types ...

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