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: $\ce{NF3}$ &lt; $\ce{NCl3}$ &lt; $\ce{NBr3}$ &lt; $\ce{NI3}$
The reason given was that more electronegative halides attract negative charge &amp; hence reduce its availability.

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

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

$\ce{NCl3}$ 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 &amp; basicity of two or more compounds(both organic &amp; 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 &amp; other people.
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.