I am a long-time reader of this forum, but this is my first time post. I am a chemistry undergrad with a question about Lewis acidity or basicity of Group 15 compounds.

I know Lewis acids act as an electron pair acceptor and a Lewis base as an electron pair donor. Then sometimes a dative bond is formed between the two, forming a Lewis adduct (as per classic $\ce{NH3}$ and $\ce{BF3}$ reaction). But this is where my knowledge becomes a little sketchy, and hence my question:

When considering group 15, is it possible for the elements of this group to form compounds which are either Lewis acids or bases?

Firstly, the requirements for Lewis acids of group 15, is it empty d-orbitals or an incomplete octet which give a compound the ability to accept two electrons?

Therefore, would this make $\ce{PF5}$ an acid (d-orbitals)? Would $\ce{AsF5}$ be classed as an acid? (If so, is this due to $\ce{As}$'s ability to violate the octet rule coupled with the fact $\ce{As}$ is very positive in this compound).

Then are the requirements for something to be a Lewis base purely the presence of two non bonding electrons? Would this then make compounds such as $\ce{AsF3}$ and $\ce{PF3}$ bases?

I am a little lost, would like to know if the above is along the right lines. Do things like electronegativity and atom size play a part?

I think this is a base?


I think this is an acid?


  • $\begingroup$ thanks to those who edited my post earlier, I am new to posting on this site. I also realised my tags were the wrong way around on my picture, hence the repost with correct tags on picture $\endgroup$ Commented Jul 3, 2018 at 12:53

2 Answers 2


For anyone else with a similar question above... I have been researching this for the last day or two, I have found Group 15 should be spilt into two groups - either nitrogen or heavier G15 elements when considering basicity or acidity.

Nitrogen - Trihalides

NF3 < NCl3 < NBr3 < NI3 (with NI3 as the most basic due to increasing relative electronegativity of the nitrogen). (These compounds are generally only weak bases.)

In my research I have found the greater the electronegativity of the lone pair atom in comparison to its ligands (i.e. the halides in this instance), the more free the lone pair is to participate in a dative bond to form a Lewis adduct with a Lewis acid. The more free the lone pair is the stronger the base is, hence NI3 is the strongest base out of the trihalides. This is why NH3 is such a great base as the lone pair on N is not inductively pulled towards the ligands because H is relatively less electronegative. Whereas in NF3 the lone pair is so drawn away from nitrogen this compound is sometimes not even classed as a base, hence it is the weakest of the trihalides. The reason nitrogen is on its own is because it has max coordination number 4.

P, As and Sb - Trihalides or Pentahalides

I have found halides, either tri or penta, are generally classed as Lewis acids. PX3, AsX3, SbX3 (X - F, Cl, Br or I)

First the trihalides, these act as mild Lewis acids towards trialkylamines and halides. This is even though the P, As or Sb will have a lone pair. As before, this is partly due to electronegativity reducing the availability of the lone pair.

My textbook points out PF3 is probably a little bit unusual in this group because it has empty d-orbitals (making it a weak sigma donor yet a strong pi acceptor.)

The pentahalides of heavier group 15 elements are pretty strong acids PX5, AsX5, SbX5 (X - F, Cl, Br or I)

enter image description here

The trihaldies of nitrogen are explained nicely here - https://courses.lumenlearning.com/introchem/chapter/trihalides-boron-halogen-compounds/

Also a clever link about NCl3 - http://www.chm.bris.ac.uk/motm/ncl3/ncl3h.htm

The heavier G15 information and pictyre came from a textbook Inorganic Chemistry (Weller, Overton, Rouke, Armstrong)


Lewis Acid-Base Theory

Simply put, if, in a reaction, a compound is donating electrons, then it is basic while if a compound is accepting electrons, then it is acidic. Do note that this requires some kind of electron transfer. For example, there is no acid or base in the reaction $\ce{N +N->N2}$ because neither nitrogen is more or less acidic than the other.

Group 15 halides

Now, the Group 15 compounds, interesting bunch of troublemakers, with lone pairs, empty orbitals, a wide range of oxidation states, and with the infamous element $\ce{Sb}$ known to form the most acidic compound $\ce{HSbF6}$ or fluoroantimonic acid, which I talked about a little bit here. I will focus on group 15 trihalides and discuss the relative Lewis acidity or basicity.

Frontier Molecular Orbital Acid-Base Theory

In a reaction, if a highly energetic highest occupied molecular orbital (HOMO) of a compound readily donates electrons to a lowly energetic lowest unoccupied orbital (LUMO) of another compound to form a bond, then the HOMO-compound is termed basic and the LUMO-compound acidic. Thus, highly energetic HOMO hints towards basicity while a lowly energetic LUMO hints towards acidity. Since a compound has both HOMO and LUMO, it may act as a base and an acid in different reactions.

I couldn't find a study that corresponds to this directly, so I sought out to carry it myself. If someone could corroborate this study, I would greatly appreciate it. All calculations were performed at RHF level of theory using def2-SVP basis set with geometry optimization.

Group 15 Trihalides

HOMO energies of Group 15 Trihalides

LUMO energies of Group 15 Trihalides

Fixed Group 15 Element

Since Homo energies are increasing and LUMO energies are decreasing, group 15 trihalides show a regular trend of increasing basicity and acidity (both) in the order:

$$ \ce{EF3<ECl3<EBr3<EI3} $$

I doubt if $\ce{NX3}$ will act as a Lewis acid though, because I haven't seen $\ce{N}$ form compounds with an expanded octet. However, the LUMO energies fall rapidly from $\ce{NF3}$ to $\ce{NI3}$, and the latter might just be able to act as Lewis acids as well.

Fixed Halide

More clearly visible in the graphs, the compounds show an irregular trend of slight increase in basicity (refer data below), since HOMO energies are increasing slightly, and a significant increase in acidity, since LUMO energies decrease rapidly except for chlorides, bromides, and iodides of $\ce{N}$ and $\ce{P}$:

$$ \ce{NX3<PX3<AsX3<SbX3<BiX3} $$


All energies are reported in $\pu{eV}$.

Group 15 Trihalides HOMO LUMO
$\ce{NF3}$ -14.1362 5.3066
$\ce{NCl3}$ -11.2383 1.3535
$\ce{NBr3}$ -10.4263 0.7615
$\ce{NI3}$ -9.4958 0.1229
$\ce{PF3}$ -13.0271 3.8809
$\ce{PCl3}$ -11.2461 2.1658
$\ce{PBr3}$ -10.4584 2.1765
$\ce{PI3}$ -9.7250 0.7336
$\ce{AsF3}$ -14.0260 1.5621
$\ce{AsCl3}$ -11.5523 1.2755
$\ce{AsBr3}$ -10.9275 0.8633
$\ce{AsI3}$ -9.7864 0.3888
$\ce{SbF3}$ -13.3738 0.7485
$\ce{SbCl3}$ -11.2624 0.6446
$\ce{SbBr3}$ -10.6888 0.3214
$\ce{SbI3}$ -9.8524 0.0453
$\ce{BiF3}$ -13.7663 0.4729
$\ce{BiCl3}$ -11.4354 0.3244
$\ce{BiBr3}$ -10.8085 0.0180
$\ce{BiI3}$ -9.6861 -0.2291

P.S. These calculations take a lot of time, and though I am extremely curious, I am not sure if it is very useful or if I am on the right track. Some feedback or criticism would help. I will do a similar analysis with pentahalides if you find this useful.

  • 1
    $\begingroup$ This q. is 5 years old, with OP "Last seen more than a month ago", so I'm not sure she'll respond. Pentahalides are obviously Lewis acids, and OP got that^ $\endgroup$
    – Mithoron
    Commented Jun 19, 2023 at 19:41
  • 2
    $\begingroup$ Nitrogen triiodide is indeed Lewis acid so you got something there. $\endgroup$
    – Mithoron
    Commented Jun 19, 2023 at 19:44

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