[no username inserted]

Does any pπ-dπ back bonding occur in phosphoric acid's resonance structures?
 I personally guess the structure with P-O bond in which P has a positive formal charge and O a negative charge , contain pπ-dπ back bonding. While the other structure has pπ-pπ bond which causes P=O double bond. Am I right?!

[Babcock_Hall]

http://pubs.acs.org/doi/pdfplus/10.1021/ed072p583
Frank Weinhold deals with the phosphate trianion in this paper, and he argues that the original Lewis representation is good.  I am not sure about neutral phosphoric acid.

[no username inserted]

http://pubs.acs.org/doi/pdfplus/10.1021/ed072p583
Frank Weinhold deals with the phosphate trianion in this paper, and he argues that the original Lewis representation is good.  I am not sure about neutral phosphoric acid.

I couldn't get anything useful from that. Would you please explain it yourself? And I'm asking about phosphoric acid not the phosphate anion!
 Is it right to say that the hybridization of P is sp3 , So there's no p orbital to interact with oxygen non-bonding electron pairs. Therefore there has to be a pπ-dπ bond happening and not a pπ-pπ , in the most stable resonance structure. ?
That's all I can think of 

[Babcock_Hall]

That is a tall order for someone like myself, who is not an inorganic nor a quantum chemist.  I think that there is a paradox:  On the one hand, the bonds between P and O in phosphate triesters and presumably phosphoric acid are of different lengths, suggesting different bond orders for P-O-R (R = H or alkyl) bonds versus the P-O (phosphoryl) bond.  On the other hand, my take-home message from the Weinhold work is that some compounds, such as SF₆ that appear to violate the Lewis octet rule, do not.  The S-F bonds are partially ionic, and one can draw a bunch of resonance structures to that effect.  Weinhold's paper indicates that the d orbitals are not very occupied in the sulfate anion.  Possibly one way to answer your question is to look into how occupied the d orbitals are in phosphoric acid, but we can hope that a quantum chemist drops by to clarify this point.

A number of papers cite Weinhold's work in this area, but I cannot say much more than that.
Tetrahedron Letters 49 (2008) 2095–2098.  A general description of phosphorus containing functional groups Tom Leyssens, Daniel Peeters  http://144.206.159.178/ft/1010/593167/12217551.pdf