Question

Why boron have negative3 valency?

Answer 1

Explanation:

BH4(1-) comes from reacting diborane (B2H6) with a hydride (H(1-)). Each H has -1 valence, and the B has +3. Add all the valences +3 + 4x-1 = -1, the charge on the borohydride

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It’s not clear what you mean by “compound BH4”. If you mean , it doesn’t contain boron at all, so presumably you must mean the anion, which is not a neutral compound. In that case, the negative charge on the anion comes from the electron it acquired from the associated cation, e.g. NaBH4.