The hexacoordinate complexes that the macrobicyclic ligands {(NH3)(2)sar)(2+) and {NMe3)(2)sar}(2+) (sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) form with transition metals such as Co-III, Co-II and Cu-II can adopt several isomeric structures. In this article, we have firstly employed DFT methods lo compute the relative stability of their Delta-ob(3), Delta-ob(2)lel, Delta-lel(2)ob and Delta-lel(3) isomers, as well as the activation barriers for their interconversion. In agreement with the experimental data, the results show that, in general, the different isomers of the Co-III and Co-II complexes present similar free energies, whereas the Cu-II complexes show a strong tendency towards the lel(3) form. In addition, the interplay between the structure and stability of these species has been studied by combining shape maps with a distortion/interaction energy analysis. In contrast to the geometries close to the ideal octahedron that all the studied Co complexes present, the le)3 structures of [Cu{(NH3)(2)sar}](4+) and [Cu{(NMe3)(2)sar](4+) are better described. as trigonal prisms. In such structures the ligand adopts a conformation significantly more stable than in the other isomers, and this drives the formation of lel(3)-[Cu{(NH3)(2)sar}](4+) and lel(3)-[Cu{(NNe3)(2)sar}](4+). Overall, the results show a clear relationship between the stability of a given isomer and its degree of distortion with respect to the ideal octahedron (or trigonal prism), with the latter being ultimately dependent on the transition metal and its radius.
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