VSEPR calculation for H₃N→BF₃

Since BF₃ is deficient two electrons from the octet, a characteristic reaction is to react with lone pair donors. Acceptance of a pair of electrons makes BF₃ a Lewis acid. The electron pair donor NH₃ is a Lewis base. The product of the reaction between NH₃ and BF₃ is sometimes written H₃N→BF₃. There are two atoms of interest, N and B. As in any other molecule containing more than one 'central' atom, it is necessary to execute a calculation for each centre.

H₃N→BF₃

Lewis structure:
Central atom:	boron
Valence electrons on central atom:	3
3 F each contribute 1 electron:	3
NH3 donates 2 electrons in dative bond:	2
Total:	8
Divide by 2 to give electron pairs	4
4 electron pairs:	tetrahedral geometry for the four shape-determining electron pairs
Central atom	Nitrogen
Valence electrons on central atom:	5
3 H each contribute 1 electron:	3
BF3 donates 0 electrons in dative bond	0
Total:	8
Divide by 2 to give electron pairs	4
4 electron pairs:	tetrahedral geometry for the four shape-determining electron pairs

The geometry of H3N→BF3. You can use your mouse to manipulate the molecule in the right hand "Jmol" image.

An alternative representation (below) of BF₃NH₃ places a positive charge on nitrogen and a negative charge on fluorine. The octet rule is still satisfied for each atom and it is now not necessary to write in an arrow to distinguish the B-N bond from other single bonds. In some ways this is an advantage since it removes the impression that a dative bond is different from other bonds. The VSEPR calculation works perfectly well on this representation.