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VSEPR calculation for H3NBF3

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 H3NBF3. 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.

H3NBF3
Lewis structure: H3N-BF3.gif
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
H3N-BF3
The geometry of H3NBF3. 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.

H3N-BF3-alt.gif

A VSEPR tutorial on the WWW

VSEPR tutorial on the WWW, URL: http://winter.group.shef.ac.uk/vsepr/
Copyright 1996-2015 Prof Mark Winter [The University of Sheffield]. All rights reserved.
Document served: Monday 29th May, 2017