Apply the VSEPR model to predict basic shape

 

•	The VSEPR model can be applied systematically by following a few simple steps. As an example we will consider the structure of hexafluorophosphate, [PF6]-.

•	First, draw the Lewis structure of the molecule and identify the central atom. For [PF6]- the central atom is a phosphorus.

 

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Next, determine the number of valence electrons on the central atom. Phosphorus has the electron configuration: 1s22s22p63s23p3 and thus has 5 valence electrons. Add to this the number of electrons that are contributed by all directly bonded atoms. For [PF6]- there are 6 fluorine atoms, each contributing 1 electron. Finally, we add one electron to account for the single negative charge on the phosphorus. This brings our total to 12.

•	Divide this number by 2 to give the total number of electron pairs (i.e. 6) and assign the coordination geometry corresponding to this number using the table in the previous section.

•	Hexafluorophosphate, [PF6]- therefore has an octahedral geometry. To summarise:

 

hexafluorophosphate, [PF6]-.
Central atom:	phosphorus
Valence electrons on central atom:	5
6 F atoms, each contribute 1 electron:	6
Add 1 for negative charge on phosphorus	1
Total:	12
Divide by 2 to give electron pairs:	6
6 electron pairs:	Octahedral geometry for the 6 shape-determining electron pairs

 

•	Check this for yourself by examining the crystal structure of this molecule (CSD refcode WINFAA). Measure the F-P-F bond angles in the structure and see how they correspond to the ideal octahedral angles given in the table in the previous section.

•	Apply the VSEPR model in order to predict the geometry of the following molecules. Confirm that your answers are correct by examining the corresponding crystal structures. Comment on how closely the observed bond angles agree with the expected ideal values.

•	[BrF6]- : CSD refcode ZAQBIC

•	[I3]- : CSD refcode RIKTAG

•	In(CH3)3 : CSD refcode TRMEIN03

•	[BeF4]2- : CSD refcode KIPPEE

•	[NH4]+ : CSD refcode ACARBM01

•	Fe(CO)5 : CSD refcode FOJBOV01

•	[SbF6]- : CSD refcode FUJLAX