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?