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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.

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  • $\begingroup$ 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
  • $\begingroup$ @DHMO I see. But anyway just for possible future reference, I want to know a source of the data. Thanks again:) $\endgroup$ – NightMare Jan 12 '17 at 15:33
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    $\begingroup$ BTW, how would one actually measure the pH+ of NI3? It would be a bang up experiment! $\endgroup$ – DrMoishe Pippik Jan 13 '17 at 0:52
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Electronegativity must be taken into account here. Hence nitrogen trifluoride will be least basic.

There are lot of things that one should just accept in chemistry rather than reasoning and thinking a lot.

I am not saying thinking is bad, I'm just saying that you can save this thinking for more logical subjects like Physics or Maths :)

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    $\begingroup$ We do a lot of things logically in chemistry, and the only times logic fails is when a minor factor dominates. So please support your answer with pkB values if you have them $\endgroup$ – YUSUF HASAN Dec 31 '18 at 17:58
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In case of Boron, boron has vacant orbitals which leads to pi back bonding from halides to boron and that concept is used to explain the basicity of Boron. However Nitrogen doesn't have vacant orbitals, hence that pi bonding is not possible from halide to Nitrogen. Consequently electronegativity is used for explanation for basicity of Nitrogen Halides.

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Hmm I think that NF3 is the most basic nitrogen trihalide .Nitrogen fluoride is the most polar molecule from all of them (NF3,NCL3,NBR3,NI3) so some areas inside the molecule will be negaitively charge which will attract protons.

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  • $\begingroup$ Nitrogen has 3 bonding pairs in NX3 where X is a halide and 1 lone pair.According to VESPR the lone pair will repel the bonding pairs more than they repel themselves so an ammonia shaped molecule will be created. $\endgroup$ – Warrior Jun 29 at 23:54

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