The structural properties of the four-coordinate dinickel(II) and the (4 + 1)-coordinate dicopper(II) compounds of two bis(diamine-diamide) ligands, based on the condensation of 1,8-diamino-3,6-diazaoctane or ethane-1,2diamine with bismalonic esters (that is the complexes of the bis-13-membered macrocyclic ligand L 1 and of the corresponding open-chained ligand L 2 ) are studied by X-ray crystallography, EPR spectroscopy, and molecular modeling. X-ray crystallography indicates that Cu 2 L 1 (OH 2 ) 2 and Cu 2 L 2 (OH 2 ) 2 have a stretched conformation (torsion angle φ(M-C bridgehead -C′ bridgehead -M′) ) 180°); Ni 2 L 2 has the same stretched conformation, but Ni 2 L 1 is folded (φ(M-C bridgehead -C′ bridgehead -M′) ) 55°). The dicopper(II) compounds have the same stretched structure in solution (MM-EPR), and molecular mechanics studies (strain energy as a function of the torsion angle φ) indicate that the most stable conformations are those observed in the solid state and in solution, i.e., stretched for Cu 2 L 1 (OH 2 ) 2 , Cu 2 L 2 (OH 2 ) 2 , and Ni 2 L 2 and folded for Ni 2 L 1 . Reasons for the stabilization of the observed structures are discussed in detail.
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