Macrocyclic nickel(II) complexes with a folded conformation. Synthesis, properties and structures of (NiL(en))(Ni(en)3)(ClO4)4 and ((NiL)2((micro)-ox))(ClO4)2 (L=3,10-bis(2-hydroxyethyl)-1,3,5,8,10,12-hexaazacyclotetradecane, en=ethylenediamine, ox=oxalate anion)/