Group 15... Lewis Acid, Lewis Base or Not At All?
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 &lt; NCl3 &lt; NBr3 &lt; 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) 



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)