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Crystallographic results retrieved from the Cambridge Structural Database (CSD) have been used to perform a systematic conformational classification of free and metal-coordinated unsaturated 15-membered oxa and thia macrocycles using symmetry-modified Jarvis-Patrick cluster analysis. Relative molecular mechanics energies of the observed conformations are compared with the cluster populations. With oxa donors a uniangular and a [348] conformer predominate for larger metal ions; these lie above the donor atom plane with 1-6 additional ligands bound on the same side. With smaller cations an anangular conformer is adopted, the O atoms describing the equatorial plane of a pentagonal bipyramid. Other conformers occur as dictated by the coordination environment, particularly if not all donor atoms are metal-bound; in some cases the conformation is determined by a hydrogen-bonded network. In some thia examples the ligand binds to an axial/apical and four equatorial sites of the coordination polyhedron; in others containing AuI or AgI the metal is linearly or tetrahedrally coordinated with additional M-S interactions. With mixed donors, the hard/soft characteristics of the metal determine the coordination mode.

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