Question

why metal hydrides are powerful reducing agents?

Answer 1

Hydrides contain hydrogen in the negative oxidation state, or in the case of some transition metals, in a co-valent state. But hydrogen “likes” to be in a positive oxidation state, that is, it is chemically more like a metal than like a non-metal. The potential for its oxidation is high, and H-minus looses electrons- the essence of reduction.For example, with sodium hydride, the H-minus donates an electron to water, which then forms OH-minus and hydrogen, and this all takes place with explosive violence.

Answer 2

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The reaction mechanism for metal hydride reduction is based on nucleophilic addition of hydride to the carbonyl carbon. In some cases, the alkali metal cation, especially Li+, activates the carbonyl group by coordinating to the carbonyl oxygen, thereby enhancing the electrophilicity of the carbonyl.
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