Oscail Tutorials

Hypervalence and the Octet rule
 
The presence of Hypervalence can be examined using ELF Basin analysis using Moilin / Orca / MultiWFN

Moilin can produce an automated a TOPMOD type ELF Analysis and Basin Plot files with just one click.    

The OCTET RULE should obeyed by p-block compounds and there are very few exceptions

A statement of the Octet Rule is -

In compounds, s and p-block elements tend to have a maximum of 8 electrons in their valence shell and a duplet on hydrogen.

The Lewis Structures for PCl₅, SO₃, SO₄^2- that are often seen in textbooks (first row below) are wrong in that they suggest that
P in PCl₅ S in SO₃ and S in SO₄^2- have 10, 12 and 12 electrons in the valence shell of the central atom.

Taking PCl₅ as an example build the molecule in Moilin.

Click the Orca icon and on the Orca menu click new input and click run and Yes.
When the message


appears click OK and click End Orca to return to the main Moilin menu.
Click the MWF button on the Moilin toolbar and the MultiWFN dialog appears


Select the third option and click OK to start Basin Generation, ELF analysis and Plot file generation..


The Wait for Calc. message remains until the calculation is complete. The listing file
will open in notepad++


Look for the table with the basins sorted with labels. Labels starting with C are core basins and
Basins starting with V are valence basins. V(Cln) indicates a lone pair on Cl. The last five
basins are the basins for the P-Cl bonds. The total number of valence electrons on P is the
sum of the populations for these 5 basins which is 7.3557.
This is less than an Octet thus PCl₅ is not hypervalent and it is best represented by the
octet obeying structure in the second row (and its four other resonance forms). 
SO₃ and SO₄^2- also have less than an octet and are best represented by ionic forms.

PPh₅ is a rare exception it is hypervalent with close to 11 electrons in the P valence shell
and this is attributed to the small electronegativity difference between P and C.

Ref DOI:10.1021/ic011003v

Plotting ELF Basins.

To plot the ELF Basins Start VMD

 

Type the the commands shown in the VMD command window each followed by return.
The files for VMD are copied to c:\moilin\scratch this saves typing long paths. 
The first command uses the cd command to change to the c:\scratch\moilin  folder. N.B. The VMD commands are case sensitive and / is used not \.



This picture should appear in the VMD window.
Note 1-synaptic and 2-synaptic basins are colored by green and red, respectively, while other basins (corresponding to core basins in this example) are colored by white.

Consult the MultiWFN manual section 4.17.10 for further information.