In the formation of B(Me)3 the borane acts as a Lewis acid because it accepts electrons. The metyl groups act as a Lewis base because of its electron donation. (reference: 2)
Lewis' theory
According to Lewis'
theory, an acid accepts a pair of electrons and a base donates
a pair of electrons. In organic chemistry, Lewis' theory is very important.
For example:
In the reaction of water with hydrogenchloride, water is a lewis base
because it donates a pair of electrons.
In the formation of B(Me)3 the borane acts as a Lewis acid because
it accepts electrons. The metyl groups act as a Lewis base because of its
electron donation. (reference: 2)
