The neutral, bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) coordinates to Cu(l) halides via the imine nitrogen, and the complexes are stabilized by intramolecular (NHX)-X-... interactions (X = Cl, Br) or are partially dissociated (X = l) in the solid state; in all cases, fluxional behavior is observed in solution. Selected complexes have been tested as catalysts for the polymerization of methyl methacrylate.