Synthesis and structure of copper(I), silver(I) and zinc(II) amides [Cu2(mpsa)2, [Cu2X2 (mpsa)4] (X = Cl or Br), [Ag4 (mpsa)4] and [(ZnEt)2 (mpsa)2] [mpsa = 2-n (SiMe3)-C5H3N-6-Me]