Mixed element hydrides

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

Figure 14.10—Hydride reactor. Used for some elements, this device includes a mixing tube where the hydride of a metal (or non-metal) is formed by a reaction with sodium borohydride. The flow of argon extracts the metal hydride that is formed (gas separator) and brings it into a quartz tube heated at 800 to 1000 degrees in the flame.

The same group also showed that mono(cyclopentadienyl) mixed hydride/ aryloxide dimer complexes of several lanthanide elements (Y, Dy, Lu) could be synthesized easily by the acid-base reaction between the mixed hydride/alkyl complexes and an aryl alcohol [144]. These complexes reacted with C02 to generate mixed formate/carboxylate derivatives, which were moderately active initiators for the copolymerization of C02 and cyclohexene oxide, without requiring a co-catalyst. The lutetium derivative 21 was the most active (at 110°C, TOF = 9.4 h ), yet despite a good selectivity (99% carbonate linkages), the molecular weight distribution remained broad (6.15) (Table 6). 

The berkelium monopnictides have been prepared on the multimicrogram scale by direct combination of the elements (138). In all cases, the lattice constants of the NaCl-type cubic structures were smaller than those of the corresponding curium monopnictides but comparable to those of the corresponding terbium compounds. This supports the semimetallic classification for these compounds. One additional report of BkN has appeared (139). The lattice parameter derived from the sample exhibiting a single phase was 0.5010 0.0004 nm, whereas that extracted from the mixed-phase sample of BkN resulting from incomplete conversion of a hydride was 0.4948 0.0003 nm. Clearly, additional samples of BkN should be prepared to establish more firmly its lattice constant. 

As with several other elements, notably Mg and Al, there are close similarities between the alkyls and hydrides, especially in the complexes with donor ligands. For the polymeric alkyls, especially BeMe2, strong donors such as Et20, Me3N, or Me2S are required to break down the polymeric structure. Mixed hydrido alkyls are known thus pyrolysis of diisopropylberyllium gives a colorless, nonvolatile polymer ... 

As shown above, one possible strategy to lower the absolute value of reaction enthalpies in lightweight hydrides is to look for elements, which exhibit negative heats of mixing with the hydride-forming elements/compounds and thus stabilize the dehydrogenated state. 

In addition to these single-phase hydrides multiphase mixed hydrides or so-called reactive composites attract increasing attention. By addition of another element or compound the reaction enthalpy of the hydride can be tailored very efficiently if a stable compound is formed from the hydride and the additive(s) upon desorption. Unfortunately, this approach leads to an unwanted decrease in hydrogen storage capacity. 

Several types of atomization cell are available flame, graphite furnace, hydride generation and cold vapour. Flame is the most common. In the premixed laminar flame, the fuel and oxidant gases are mixed before they enter the burner (the ignition site) in an expansion chamber. The more commonly used flame in FAAS is the air-acetylene flame (temperature, 2500 K), while the nitrous oxide-acetylene flame (temperature, 3150K) is used for refractory elements, e.g. Al. Both are formed in a slot burner positioned in the light path of the HCL (Fig. 27.4). 

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