Why Do SO2 and XeO4 Form Different Types of Bonds?

[asdff529]

The S molecule of SO2 forms a double bond with one O and dative covalent bond with another O
Meanwhile,XeO4 is a molecule which Xe forms a double bond with each O
I have a question here
Why doesn't that S forms 2 double bonds with O to extend its octet structure?
Similarly,why doesn't that Xe form 4 dative covalent bond with O to maintain its octet structure?

 

[DrDu]

Neither sulfur nor xenon extend their octet in these compounds and a distinction between double bonded and dative bonded oxygens is purely fictive.

 

[asdff529]

Neither sulfur nor xenon extend their octet in these compounds and a distinction between double bonded and dative bonded oxygens is purely fictive.

why do u say that "Neither sulfur nor xenon extend their octet in these compounds"
if i draw the electron diagram of Xe,i count there should be 16 outermost electrons around Xe?

 

[DrDu]

Bonding in these compounds is quite complex. Maybe you can find in your library
http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0471972428,descCd-tableOfContents.html

 

[asdff529]

Bonding in these compounds is quite complex. Maybe you can find in your library
http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0471972428,descCd-tableOfContents.html

i think I am not able to understand it
btw,thx a lot

 

[asdff529]

but can i explain like this:
since electrons are indistinguishable,and they all move randomly around the atoms
so it is indistinguishable whether it is dative or pure covalent?

 

[DrDu]

Maybe this wikipedia article is helpful:

http://en.wikipedia.org/wiki/Hypervalent_molecule