Question

Is B(OH)3 a lewis base?

Answer 1

`"B"("OH")_3` has a trigonal planar structure:

And there are no lone pairs of electrons in the boron's central `2p_z` orbital. It is an empty unhybridized orbital, like this (except with `"OH"` instead of `"H"`):

Therefore, in order to bond, boron acts as an electron acceptor and takes electrons into its empty orbital, following the definition of a lewis acid.

(Afterwards, it experiences a conformational change and becomes a tetrahedral structure.)