# Order of Basicity of Nitrogen Trihalides

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.

• 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...
– DHMO
Jan 12 '17 at 15:11
• @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. Jan 12 '17 at 15:22
• BTW, how would one actually measure the pH+ of NI3? It would be a bang up experiment! Jan 13 '17 at 0:52
• The comment was just meant in humor... NI3 is fun to create in minuscule amounts, but any manipulation ends in a bang. Jan 13 '17 at 8:24
• 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 Jan 1 '19 at 0:59