It is interesting that trivial notions used to describe the valency of elements may become sometimes tricky. Let us take the example of phosphoric acid (H3PO4), phosphorous acid (HO)2PHO or, alternatively (HO)3P and hypophosphorous acid HOPO2H2. The coordination number is always 4 (with the exception of (HO)3P, which exists only in some cases, although even there one can find a fourth ligand, a lone pair). Using classical bond picture the valency is always 5 (one oxygen is in double bond, again with the exception of (HO)3P), the oxidation number is somewhat ambivalent, as the electronegativity of BOTH hydrogen and phosphorus is 2.1. The general rule is that the bonding pair is ascribed to the more electronegative partner. If one takes the oxidation number of hydrogen +1 in each case, one will get an oxidation number of +5, +3 and +1 for phosphoric acid, phosphorous acid and hypophosphorous acid respectively. I just wonder whether in qunatum chemical calculations what is the actula polarity of the P-H bonds in phosphorous and in hypophosphorous acids?
Ah, you have hit upon a real truth, that in the most general sense oxidation states are somewhat arbitrary in nature as they are derived from a purely ionic model of interaction based a non-fixed parameter, electronegativity. We must remember that the real "electronegativity" of an atom in a molecule depends intimately on what else is attached. From this perspective, the electronegativity of P increases as more oxygens are attached as they strongly inductively remove electron density from the P center. to your final point, my guess is that computational approaches will capture the reality and the differences to a varying degree depending on the level of calculation and the assumptions used in calculating atomic charge ("natural" vs Mulliken vs. etc).
Ah, you have hit upon a real truth, that in the most general sense oxidation states are somewhat arbitrary in nature as they are derived from a purely ionic model of interaction based a non-fixed parameter, electronegativity. We must remember that the real "electronegativity" of an atom in a molecule depends intimately on what else is attached. From this perspective, the electronegativity of P increases as more oxygens are attached as they strongly inductively remove electron density from the P center. to your final point, my guess is that computational approaches will capture the reality and the differences to a varying degree depending on the level of calculation and the assumptions used in calculating atomic charge ("natural" vs Mulliken vs. etc).
Gentlemen,
in addition to the excellent Professor Bachman's answer, here are the results of natural charge quantum chemical calculations (B3LYP/6-311+G*) for the acids:
Hypophosphorous acid [HOP(O)H2]:
charge on phosphorus P: +1.7,
attached hydrogens: -0.1,
attached oxygen O: -1.0,
oxygen from OH group: -0.97,
H from OH: +0.50
Phosphorous acid [HPO(OH)2]:
charge on phosphorus P: +2.1,
attached hydrogen: -0.1,
attached oxygen O: -1.0,
oxygens from OH group: -0.97, -0.98,
H from OH: +0.50, +0.50
So, in Phosphorous acid P-H bonds are more polar than P-H in Hypophosphorous acid.
Dear colleagues, thank you for the comments. Actually I did not ask originally for help, it was the modification of the editors of the website. I was simply recognizing the problem and pondered on the solution. Thank you for your explanatins and added data.
Dear Mr. Alexander Ivanov, would you plz tell me what is the charges you've reported based on?I mean NBO analysis or anything else. tnx.
The following "hold" paper could b of soma help
X-OH and O-H...O bond lengths in protonated oxoanions. Acta Crystallographica B40, 1-6 (1984).
We recently addressed the point in the article attached. The electronegativity considerations are of fundamental importance and may provide indications, which not necessarily are in agrement with what written in textbooks.
Many thanks. Wow, that is a rich work! I'll take time to read it. Thanks again.
To: Gyorgy Banhegyi
When I read your question, I thought that you meant the normal and known oxyacids of phosphorous. In my book, Encyclopedia of the Alkaline Earth Compounds published last year by Elsevier, I gave a brief summary of the properties of these compounds. I am unaware of any acids in which P^+4 exists. The following lists all of the known oxyacids in which P^+5 exists:
4.2.1. - Phosphorous Oxy-Acids
4.2.1.1. - Phosphorous Oxide Anhydrides
Diphosphorous Trioxide
Phosphorous(III) Tetraoxide (P4O6)
Phosphorous(V) Pentoxide
4.2.1.2. - Phosphorous Acids
Hydrogen Phosphide as an Acid
Hypophosphorous Acid
Metaphosphoric Acid
Pyrophosphoric Acid
Hypophosphoric Acid
Peroxy-phosphoric Acid
Tripoly- and Tetrapoly-phosphoric Acid
Hypophosphorous Acid
Phosphorous Acid
Phosphoric Acid
Metaphosphoric Acid
Pyrophosphoric Acid
Hypophosphoric Acid
Peroxo-phosphoric Acid
Tripoly- and Tetrapoly-phosphoric Acid
I hope that this is useful in your work. - RC Ropp
Dear Dr Ropp - thanks for your comment. I did not say that the oxidation number of P is 4, I said that the COORDINATION number is 4 (in most of the listed cases).
To Carlos Mealli,
Very nice paper. It reminds me of work from long ago (my PhD) that considered the nature of the lone pair on a bismuth with a mix of organic an low-valent transition metal carbonyl groups attached. In that work, we found that the surrounding "environment" including the potential binding partners played a significant role in the behavior of that lone pair (stereochemically inert or active). I think, because we often drawn species in a vacuum (isolated molecules on a white page), we too often forget that observed structure and behavior of molecules is always contextualized by the environment.
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