The boron atom actually has an empty p orbital, which makes it act as a strong Lewis acid (an electron-pair acceptor). It reacts readily with a Lewis base (an electron-pair donor) such as ammonia.

The bond that is formed between the boron atom and the nitrogen atom is called a coordinate covalent bond because the nitrogen atom supplied both of the electrons for the bond. There is a positive formal charge on the nitrogen atom and a negative formal charge on the boron atom. This type of bond is sometimes represented with an arrow that points away from the atom that donated both electrons and the charges are not shown.