Reaction with the Hydride Ion

Hydrides can be categorized in various classes according to the nature of the bond that links the metal and hydrogen. The following categories can usually be found in the literature ionic hydrides (hydrides with group 1 and 2 elements), covalent hydrides, polymeric hydrides, metallic hydrides, borderline hydrides, and transitirai metal complex hydrides. Here the reaction of ionic hydrides with carbon dioxide (CO2) is considered. Transition metal hydrides with the formula L M-H are discussed in Chap. 4, where the interaction of CO2 with analogous E-X systems (where E and X are different elements) is discussed. 

Group 1 and group 2 hydrides are ionic, insoluble in aprotic solvents, and not usable in protic solvents such as water and alcohols because they promptly react with H2 evolution  

The ionic hydrides are thermally very stable and have quite high melting temperatures. Depending on the element, they can melt without (CaHa) or with (LiH, NaH, and others) decomposition. NaH decomposes above 573 K, whereas LiH and CaH2 are stable up to 900 K. 

Therefore, the reaction of ionic hydrides with CO2 mostly implies a heterogeneous, biphasic system in which pressurized CO2 reacts at a temperature of 473 K with the MH (M = Li, H, K, Rb) or MH2 (Ca, Ba, Sr) species to afford the relevant formate  

Aresta et at, Reaction Mechanisms in Carbon Dioxide Conversion, DOI 10.1007/978-3-662 6831-9 3 

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