. Can. J. Chem. 69,687 (1991). The crystal structures of three crown ether complexes (cyclohexano-15-crown-5.2LiOPh (I): Li207C26H36, Mr = 474.45, h(CuKa)filt. = 1.5418 A; cyclohexano-15-crown-5.2NaOPh (2): Na207C26H36, MI = 504.54, h(CuKa)filt. = 1.5418 A; and 15-crown-5.2NaOPh (3): Na207C22H30, MI = 452.46, h(CuKa)mono. = 1.5418 A) have been determined by X-ray diffraction at room temperature (T = 298 K). Complex 1 crystallizes in space group P2,/n with a = 15.032(5), b = 13.332(2), c = 13.533(3) A, P = 106.28(2)", V = 2603.2 A3, andZ = 4. Complex 2 crystallizes in space group P2Jn with a = 15.94(1), b = 12.194(7), c = 14.068(7) A, P = 102.84(5)", V = 2666.3 A3, andZ = 4. Complex 3 crystallizes in space group P2,/c with a = 12.451(3), b = 17.263(3), c = 12.659(6) A, P = 1 15.74(3)", V = 2450.8 A3, and Z = 4. The three structures were solved by direct methods and refined by full-matrix least-squares calculations to residuals, R, of 0.080,0.069, and 0.081 for complexes 1, 2, and 3, respectively. In each structure the macrocycle forms a 1:2 host-guest complex with the alkali metal. Each structure exhibits a dimer of the general form (MAC.2MOPh)2 where M = Li+ and Na+ for structures 1 and 2, respectively, and MAC = cyclohexano-15-crown-5, while M = Na+ and MAC = 15-crown-5 for structure 3. In each case the two metals of the monomeric unit have different crystallographic and chemical environments. In structures 1 and 3 one alkali metal is coordinated to all five oxygens of the macrocycle and to one of the phenoxide oxygens, while the other alkali metal is coordinated to only one oxygen of the macrocycle and to three phenoxide oxygens. In structure 2, one alkali metal is coordinated to only four of the five oxygens of the macrocycle and to two of the phenoxide oxygens, while the other alkali metal is coordinated to only one oxygen of the macrocycle and to three phenoxide oxygens (as in 1 and 3).Key words: crown ether, host-guest complexes, X-ray crystallography Dans chacune des structures, le macrocycle forme un complexe 1 : 2 hBte-invitC avec le mCtal alcalin. Chaque structure comporte un dimkre de la forme gCnCrale (MAC.2MOPh)2; pour les structures 1 et 2, MAC = cyclohexano-15-couronne-5 et M = Li+ et Na' respectivement alors que M = Na+ et MAC = 15-couronne-5 pour la structure 3. Dans chaque cas, les deux mCtaux de l'unitC monomkre sont dans des environnements cristallographiques et chimiques diffkrents. Dans les structures 1 et 3, un mCtal alcalin est coordinC aux cinq oxygknes du macrocycle et un oxygkne phenolique alors que l'autre metal alcalin est coordinC 2i un seul oxygkne macrocyclique et i trois oxygknes phenoliques. Dans la structure 2, un metal alcalin est coordinC 2 seulement quatre des cinq oxygknes du macrocycle et i deux oxygknes phCnoliques alors que l'autre metal alcalin est coordinC a seulement un oxygkne du macrocycle et i trois oxygknes phCnoliques.