Why does a hydride like NaH act as a base, but a hydride like NaBH4 act as a nucleophile?

Question

A TA talked about this today in the context of ketones. If you stick an acetone and $\\ce{NaBH4}$ together in a protic solvent, you get 2-propanol. If you stick acetone and $\\ce{NaH}$ together, you get an enolate. What's the difference? They're both hydrides.

Answer 1

NaH has hydride ion, and it is a base. It is also a reducing agent and it can reduce main group compound. 6 NaH + 2 BF3 → B2H6 + 6 NaF. But it is not commonly used as reducing agent. When it reacts with acetone or other carbonyl compounds, it removes one of the α hydrogen, enolate is formed.

In case of NaBH4, B-H sigma bonding orbital acts as HOMO, there is no hydride ion. So it is a reducing agent.