Preparing an amine from an azide
I googled it for a while and this was the best thing I found (Source):


  This is a tough question, that is probably not known with any certainty.
  Even Jerry March (Adv Organic Chem, 4th ed, page 1219) doesn't commit himself to a mechanism.
  
  You'll have to speculate.   Add a hydride on the N next to the alkyl group, then
  grab a proton on the terminal N.    Add a hydride on the terminal N, then grab a proton on the middle N.  After that, it gets a little sketchy for breaking the N-N bond.    
  
  Try a SN2 attack by hydride on the middle N, giving hydrazine and  RNH-, which is a lousy SN2 leaving group, but we're stuck with it.
  Protonation of the RNH- gives the amine and hydrazine.   Or, you could do a SN2 on the N next to the alkyl group, and kick out a NH2NH- anion, which is also a lousy leaving group for the SN2.
  You can see why most books conveniently leave out a mechanism for this reduction.