What is the nature of the bonds between P and O in oxoacids of phosphorous?

Is it a double bond or coordinate bond?

Wikipedia says that it is "double bond" but many times I encountered "coordinate bond".

  • $\begingroup$ Could you perhaps link to the specific wikipedia page that says it's a double bond? Just because there are many pages for different oxoacids of phosphorous and to avoid any confusion. $\endgroup$ – Jerry Jan 4 '14 at 18:26

In general, I'm not sure double bond and coordinate covalent bond are mutually exclusive labels and depending on the specific chemical you're looking at, they might both apply.

My Reasoning:

  1. Coordinate bonds are bonds where both electrons were donated by one atom. Usually, the concept of a double bond is used in the context of valence bond theory and coordinate bond is used in the context of crystal field theory. There is evidence for pi bonding in coordinate complexes (i.e. back bonding) leading to what could be called a double bond in valence bond theory.

  2. If you look at bond length data you can usually determine if there is a double bond. (Or if you have some of the acid and access to an IR, you can do this yourself.) The CRC should contain reference bond lengths.

I 'm not an expert in phosphorous chemistry, but these could be helpful:

  1. Structural calculations for many compounds, including a few acids

  2. Study on structures of phosphates, not acids


The phosphoryl group is usually written with a double bond, i.e. P=O, which is easier to use to describe chemical mechanisms (and also clearer on a scheme). It is also drawn like a coordinate bond sometimes. The reasoning is that it is formed from a trivalent phosphorus compound (PR3) which has a lone pair of electrons on the phosphorus, and an atom of oxygen which misses 2 electrons to complete its outer shell; and this is the definition of a coordinate bond.

The most admitted use in the scientific literature is to draw a double bond.


Numerous oxoacids of phosphorus their respective ions or salts are known.Some points which could be enumerated from all the structures are:

  • All phosphorous atoms in the oxoacids and the oxoanions are 4-coordinated and consists of at least one P=O bond.
  • All the phosphorous atoms in the oxoacids contain at least one P-OH bond which is ionizable.
  • Some oxoacids have one direct P-H bond which is non-ionizable
  • Catenation takes place via P-O-P links or direct P-P bonds. The former either exists in linear chains or cyclic species.
  • $\begingroup$ my question is regarding to first point. Sometimes I saw also coordinate bond instead of double bond, can't decide which is correct. $\endgroup$ – ankit Jan 7 '14 at 12:22

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