I was asked to arrange nitrogen tri-halides in order of increasing basicity. On looking up the answer online, it turned out to be the following: $\ce{NF3}$ < $\ce{NCl3}$ < $\ce{NBr3}$ < $\ce{NI3}$

The reason given was that more electronegative halides attract negative charge & hence reduce its availability.

I thought that the order should be $\ce{NF3}$ > $\ce{NI3}$ > $\ce{NBr3}$ > $\ce{NCl3}$ because:

  1. all the halides except fluorine have vacant d-orbitals. So, back-bonding should reduce the basicity of all but $\ce{NF3}$.

  2. $\ce{NCl3}$ shows more effective back-bonding as compared to $\ce{NBr3}$ which shows more effective back-bonding than $\ce{NI3}$. Better the back-bonding, lesser is the availability of the lone pair on the nitrogen atom.

It seems to imply that electronegativity is a more dominant factor as compared to back-bonding. But when we compare the acidity of boron tri-halides, we consider back-bonding to be the dominant factor. Same goes for Ipso effect.

So, what is wrong with my assumptions?

PS: I got the order from Yahoo! answers(which is pretty unreliable BTW)

PSS: I often come across questions of this sort which ask me to compare acidity & basicity of two or more compounds(both organic & inorganic). I think of looking at the pKb/pKa values of the chemicals can help me to reliably get the order in some cases. But I couldn't find any source/compilation of those values. Do you know any source which lists the pKb/pKa values of chemicals? If so, please add it to your answer. It may prove to be really useful for me & other people.

  • $\begingroup$ Which theory of acid/base are we referring to? pKa refers specifically to Brønsted-Lowry while I feel that you are talking about Lewis base... $\endgroup$
    – DHMO
    Jan 12 '17 at 15:11
  • $\begingroup$ @DHMO Yes, I am talking about Lewis bases. I see your point. But a source of pKa/pKb values would be useful nonetheless. I'll edit the question. Thanks. $\endgroup$
    – NightMare
    Jan 12 '17 at 15:22
  • $\begingroup$ The pKa/pKb values would be quite useless as they are not soluble in water. $\endgroup$
    – DHMO
    Jan 12 '17 at 15:31
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    $\begingroup$ BTW, how would one actually measure the pH+ of NI3? It would be a bang up experiment! $\endgroup$ Jan 13 '17 at 0:52
  • 2
    $\begingroup$ See, as far as I know, backbonding will be surely negligible for NBr3 and NI3 as pπ-dπ bonds between 2p-4d and 2p-5d orbitals will be quite ineffective. Also, I have got a source which says that the N-Cl bond in NCl3 is mostly non-polar, so that means there is similar electron density between these atoms, so backbonding is ruled out here as well. Hence electronegativity maybe the only dominating factor here $\endgroup$ Jan 1 '19 at 0:59

This can be attributed to the electronegativity of nitrogen and the lone pairs in halogens. Most bonds are all about energy if it releases more energy, it would be the "correct" structure/interaction. In boron, there are no lone pairs and backbonding occurs even if donor atom is more electronegative. But in case of halogens we have lone pairs and they will lessen the magnitude of bond enthalpy. Obviously we want a theoretical approach to check if the backbonding is favourable or not. So it is best to compare electronegativity difference in such cases. The halogens below Cl are less electronegative than nitrogen and would not normally show backbonding with it. In case of chlorine lone pair repulsions outweigh the electronegativity difference. In fluorine there is no vacant orbital so no backbonding. Thus backbonding isn't a deciding factor in this case.

  • $\begingroup$ I would really appreciate if the point that I missed is pointed out or is written as a separate answer. The point of a community as this is to share correct knowledge. The validity of answers can be ensured only when faults are pointed out. $\endgroup$ Sep 15 at 18:33

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