The Cu2+ and Ni2+ complexes of three reinforced tetraazamacrocycles, containing a piperazine subunit and one or two alkyl substituents at the other two N-atoms have been prepared and their structural properties studied. In solution, the NiZ+ complexes are square-planar and show no tendency to axially coordinate a solvent molecule or an N; ion. In contrast, the Cu2+ complexes change their geometry depending upon the donor properties of the solvent, being square-planar in MeNO, and pentacoordinate in DME They also easily react in aqueous solution with N; to give ternary species with pentacoordinate geometry, the stabilities of which have been determined. In the solid state, the X-ray crystal structures of three Cuz+ complexes also show both geometrical arrangements, two having a square-planar, the other one a distorted square pyramidal geometry. The difference behavior of NiZi and Cu2+ stems from the fact that the structural change from square-planar to square-pyramidal can easily be accomplished for Cu2+, whereas, for Ni2+, it is accompanied by an electronic rearrangement from the low-spin to the high-spin configuration. The relatively rigid ligands cannot adapt to the somewhat larger high-spin Ni2+ ion.Introduction. -Structural aspects of macrocyclic metal complexes have been studied in detail, and the factors governing the stereochemistry and coordination geometry have extensively been discussed. One of the main factors is the ring size. If the cavity of the macrocycle is to small to encompass the metal ion, the macrocycle has to fold in order that all four donors can coordinate the metal ion. Several examples of such structures have been described in the literature for metal complexes of 1,4,7,1 O-tetraazacyclododecanes, which have either cis-octahedral [2] or pentacoordinate geometries [3]. On the other side, if the macrocycle is large enough to accommodate the metal ion in its centre, square-planar or trans-octahedral complexes are formed. Examples have been observed in many metal complexes of 1,4,8,1 I-tetraazacyclotetradecane ( = cyclam) [4]. Enlarging the ring size even more allows to form tetrahedral species, especially when the metal ion has electronic properties which prefer this type of geometry. For example, in the series of N,S, macrocycles with different ring size a tetrahedral Cut complex is formed by the 16-membered ring [5]. In addition, it has been shown by Hancock that the selectivity patterns of tetraazamacrocycles is controlled more by chelate ring size than by macrocyclic ring size [6].