[gebbissimo]

I am wondering, why the ylide resonance structure (http://en.wikipedia.org/wiki/Ylide) is the way it is.

As e.g. the phosphor is more electronegative than the C-atom to my knowledge I would have placed the negative charge on the phosphor atom and the positive charge on the C-atom (meaning to switch the formal charges). And I can't think of an argument against my idea.

Any help is very much appreciated, thanks upfront!

[majorjp]

The simple answer is that the ylene form would violate the octet rule for Phosphorus and require hypervalancy which cannot, yet, be explained by standard bonding theory.

While the ylide form statisfies the octet rule for both the Carbon and Phosphorus atom.

[408]

HetN(+)-N(-)-NO2

A nitro imide-ylide 

So sexy...