[apaajaboleh]

How does the bond length of NH3 and boron trihalides (BF3, BCl3, etc) related to electronic changes upon adduct formation (H3N—BF3, H3N—BCl3, etc)?

I know that adduct formation results in an increase in the length of the B-X (X=F-I) bond as compared with its acid form, and the same trend can be seen in the length of the N-H bond, where the adduct is longer than NH3. In contrast, the X-B-X angles are found to be smaller in the adduct than in the acid form. But how does this relate to the electronic changes upon adduct formation? What even are electronic changes?