Tetranuclear magnesium complexes with chelating alkoxo ligands have been synthesized with the aim of investigating coordinatively unsaturated magnesium sites able to bind TiX4 (X = Cl, OR), of the type necessary for the formation of the active centers in polymerization catalysts. The magnesium compound [Mg-4(mu(3),eta(2)-ddbfo)(2)-(mu,eta(2)-ddbfo)(2)(mu,eta(1)-ddbfo)(2)(eta(1)-ddbfo)(2)]. 2CH(2)Cl(2) (1) (ddbfo = 2,3-dihydro-2, 2-dimethyl-7-benzofuranoxide) was prepared by the reaction of MgBu2 with ddbfoH in dichloromethane. Complex 1 exists as a centrosymmetric tetranuclear species with two different types of magnesium centers corresponding to octahedral MgO6 and trigonal bipyramidal MgO5 geometry. Compound 1 is monoclinic, space group P2(1)/c, with a = 12.053(2) Angstrom, b = 13.323(3) Angstrom, c = 17.069(3) Angstrom, beta = 98.50(3)degrees, and Z = 4. The reaction of 1 with methanol in tetrahydrofuran (THF) gave compound [Mg-4(mu(3)-OMe)(2)(mu,eta(2)-ddbfo)(2)(mu,eta(1)-ddbfo)(2)(eta(1)-ddbfo)(2)(CH3OH)(5)]. CH3OH . THF (2). During this reaction one of the two five-coordinate MgO5 centers in 1 is completed by a methanol molecule and becomes octahedral in 2. Species 2 belongs to the P2(1)/n monoclinic space group, with a = 13.323(3) Angstrom, b = 20.768(4) Angstrom, c = 27.584(6) Angstrom, beta = 104.26(3)degrees, and Z = 4. Compound [Mg-4(mu(3),eta(2)-thffo)(2)(mu,eta(2)-thffo)(2)(mu,eta(1)-thffo)2{mu-OTi-(DIPP)(3)}(2)]. 2CH(2)Cl(2) (3) is formed as a result of substitution of two thffo (thffo = 2-tetrahydrofurfuroxide) ligands bonded to the five-coordinate magnesium atom in [Mg-4(thffo)(8)] by bulky OTi(DIPP)(3) (DIPP = diisopropylphenolate) groups. Crystals of 3 are monoclinic, space group P2(1)/n, with a = 17.069(3) Angstrom, b = 18.421(4) Angstrom, 17.815(4) Angstrom, beta = 90.77(3)degrees, and Z = 4. The X-ray crystal structures of complexes 1-3 are discussed in terms of explaining the role of the coordinatively unsaturated magnesium site in chiral catalyst active center formation.