The key difference between LiAlH4 and NaBH4 is that LiAlH4 can reduce esters, amides and carboxylic acids whereas NaBH4 cannot reduce them.
Both LiAlH4 and NaBH4 are reducing agents. But LiAlH4 is a very strong reducing agent than NaBH4 because the Al-H bond in the LiAlH4 is weaker than the B-H bond in NaBH4. This makes the Al-H bond less stable. The reason for this is the low electronegativity of Aluminum compared to Boron. Therefore, the low electronegativity shifts the electron density towards the hydrogen in Al-H than that of B-H bond. As a result, LiAlH4 is a better hydride donor.
