Transition-metal hydrides hydroxides

The hydride ion (H-) is an efficient small ligand in organometallic chemistry. The first transition metal hydrides were prepared using the Hieber base reaction, exemplified in (6.50). The hydroxide adds to the carbon of one CO ligand to produce an intermediate that rapidly loses carbon dioxide, leaving the hydride ion to occupy the coordination site. 

H2 gas itself is rapidly sorbed by the transition metals and more slowly by metal oxides and elements such as carbon (graphite) and germanium. On the oxides the sorption frequently leads to the formation of hydroxides, and on heating H2O may be desorbed. Some reversible sorption occurs as well, and it has been suggested that this corresponds to a hydride formation with the surface metal ions. In the case of metals, H2 gas is sorbed rapidly even at 78 K with a heat of sorption which may be of the order of 40 Kcal or more, decreasing slowly with increasing coverage until near saturation, when it can approach zero. A considerable account of evidence supports the view that the sorption on metals is a direct 1 1 stoichio-... 

Apart from these specific hydrogen absorption and desorption properties, it is clear that the intermetallic compounds are more often used as precursors of new and more active catalysts. The decomposition of intermetallic compounds into transition-metal particles and rare-earth hydride, oxide or hydroxide in the course of the catalytic reaction was evidenced in a large number of catalytic reactions, especially those involving CO dissociation, which produces oxidizing species. The rise of catalytic activity was... 

Alkali oxides are thermodynamically stable up to very high temperatures, and even hydrides have saline character and considerable stability. Lithium nitride is a compound which can be isolated from the solution in the metal in crystalline form. Dissolved oxides have the ability to react with transition metal oxides to form complex oxides, or with hydrogen to form hydroxides of the heavier alkali metals. Lithium cyanamide is formed by means of the reaction between nitrogen and carbon dissolved in the molten metal. The reaction product in liquid sodium is sodium cyanide. 

The reaction of CO2 with a metal hydride produces formate complexes M-0C(0)H, not formyl derivatives M-C(0)0H, and the insertion into M-C bonds gives the appropriate carboxylate compounds M-0C(0)R. In a similar fashion, the reactions with M-OH and M-OR (R = alkyl, aryl) generate the corresponding bicarbonate M-0C(0)0H and carbonate M-0C(0)0R species, respectively. The reaction of CO2 with a zinc hydroxide moiety is particularly important in biological systems, namely, for the reversible hydration of CO2 to HCOs catalyzed by Zn(ll) in carbonic anyhdrases. Moreover, it has been postulated that the insertion of CO2 into M-O bonds is essential in the co-polymerization of CO2 and epoxides and in the preparation of cyclic carbonates and polycarbo-In a similar vein, the insertion of CO2 into the M-N bond of both main group and transition metal... 

The transition metals can be reduced in basic aqueous solutions via other mechanism. For example, the metal carbonyls could be attacked by the hydroxide ions and the metal reduced to metal hydride species by the elimination of carbon dioxide to yield a hydride, which could then be deprotonated with the excess of hydroxide ions (Figure 17). The reduction of the metal complexes by CO in aqueous phase is indeed a very important step in the Reppe-type catalysis and water-gas shift reactions. 

Anion displacement of halides with azolyl anions is one common route to azolyl complexes. One example of such a synthesis is shown in Equation 4.21. In other cases, azolyl complexes have been prepared by proton transfer between the free azole and a metal alkox-ide or hydroxide. An example involving the synthesis of palladium-azolyl complexes is shown in Equation 4.22. In some rare cases, reactions of pyrrole and d early metal alkyls also lead to the formation of a metal-nitrogen bond via o-bond metathesis, as shown in Equation 4.23. Finally, several late-transition-metal-azolyl complexes possessing accompanying hydride Hgands have been prepared by N-H activation of pyrrole and other azoles. 

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