Lead hydride, decomposition

The hydrogenation activity of the isolated hydrides 3 and 6 towards cyclooctene or 1-octene was much lower than the Wilkinson s complex, [RhCKPPhj) ], under the same conditions [2] furthermore, isomerisation of the terminal to internal alkenes competed with the hydrogenation reaction. The reduced activity may be related to the high stability of the Rh(III) hydrides, while displacement of a coordinated NHC by alkene may lead to decomposition and Rh metal formation. 

Reduction with lithium aluminum hydride is a useful method of identification since the respective diols are formed.12,34 Catalytic hydrogenation usually leads to decomposition of the dioxetanes into the corresponding carbonyl products.12,31 This is not surprising since transition metals promote fragmentation of these labile materials.49 In the case of dioxetane lz, the use of zinc in acetic acid was the only successful method for reducing it to its diol.31... 

Alkoxides. y6-H elimination from alkoxides is responsible for the reducing properties of alcohols towards some transition metal complexes, particularly in the presence of base. The formation of a metal alkoxide followed by /1-H elimination affords a hydride. Decoordination of the aldehyde and HX elimination, if the hydride is unstable, will reduce the oxidation state of the complex in two units, often leading to decomposition products (Scheme 6.27). 

When Y in Scheme 7.21B is hydrogen, the reverse process yielding a metal carbene-hydride complex is c -hydrogen elimination process. It provides an important route leading to decomposition of metal alkyls beside the more often encountered -hydrogen elimination pathway giving metal hydride coordinated with an olefin. 

The alkylidenemalonodinitriles shown in Table XI react exothermically in benzene at 20° C with a stoichiometric amount of -Bu3PbH to give yellowish, viscous 1 1 adducts. No decomposition of the lead hydride... 

These methods deal with specific cases. The list of examples is not exhaustive. The low-T (200-300°C) decomposition of the transition-metal borohydrides M(BH4> , e.g., leads to titanium, zirconium, halfnium, uranium and thorium borides . Alternatively, the uranium diboride may be obtained by reacting uranium hydride with diborane in hydrogen at 200-400°C. 

Some reactions of the type H+hydride - hydride radical+H2 have been studied, mainly at lower temperatures, with H atoms generated by an external source. There might be appreciable errors in extrapolation of these rate coefficients to temperatures where thermal decomposition takes place. In many cases only a lower or upper limit of the rate of consecutive reactions can be given, especially if the decomposition takes place at temperatures appreciably above 1000 °K. We will not discuss reaction mechanisms in detail which lead to untested rate phenomena nor those which are based upon product analysis without a well-defined time history. It is true, however, that no decomposition of a hydride consisting of more than two atoms has a mechanism which is fully understood and which can be completely described in terms of the kinetics of the elementary reactions. 

In contrast to the interstitial hydrides, where the metal lattice hosts the hydrogen atoms on interstitial sites, the desorption of the hydrogen from the complex hydride leads to a complete decomposition of the complex hydride and a mixture of at least two phases is formed. For alkali metal tetrahydroborates and tetrahydroaluminates, the decomposition reaction is described according to the following equation ... 

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