The preparation and characterization of a series of encapsulated-lanthanide 15-metallacrown-5 complexes are reported. Planar ligands such as picoline hydroxamic acid (picha) or nonplanar cr-amino hydroxamic acids (e.g., glycine hydroxamic acid (glyha)) led to one-step syntheses of metallacrowns in yields as high as 85%. The reaction of the appropriate hydroxamic acid with copper acetate and 1/5 equiv of gadolinium(III) or europium(III) nitrates in DMF or water yielded crystals of GcB(NO3)(3)[15-M-Cu(II)N(picha)-5], 1, Eu(NO3)(3)[15-MCCu(II)N(picha)-5], 2, and Eu(NO3)(3)[15-MCCu(II)N(glyha)-5], 3. Several other 15-metallacrown-5 complexes were synthesized with (1) Cu(II) or Ni(II) in the metallacrown ring metal position, (2) various lanthanides (La(III), Nd(III), Sm(III), Eu(III), Gd(III), Dy(III), Ho(III), Er(III), and Yb(III)) encapsulated in the center of the ring, and (3) chiral cl-amino hydroxamic acids (e.g., phenylalanine hydroxamic acid (H(2)pheha), leucine hydroxamic acid (H(2)leuha), and tyrosine hydroxamic acid (H(2)tyrha)). It is believed that all of the complexes containing Cu(II) ions have the ring metals either in four-coordinate, square-planar environments, bound to two tetradentate hydroximate ligands, or in five-coordinate, square-pyramidal geometries if solvent is bound. Spectroscopic and magnetic characterization of the Ni(II) complexes suggests that they are either five- or six-coordinate. The encapsulated lanthanides are generally pentagonal bipyramidal, with five oxygen donors from the metallacrown ring and solvent or bidentate nitrate ions in the axial positions. The circular arrangement of ions results in interesting magnetic behavior. With Dy(III) encapsulated in the center of the ring, a magnetic moment as high as 10.9 mu(B) is achieved. Analysis of the variable-temperature susceptibility of La(NO3)(3)[15-MCCu(II)N(picha)-5] indicates that the five Cu(II) ions are antiferromagnetically coupled, forming an S = 1/2 ground spin state with a moment of 1.7 mu(B) at liquid helium temperatures. Complex 1 shows ferromagnetic coupling of the Gd(III) ion to the five Cu ions at temperatures below 15 K. Studies of the metallacrown complexes in solution show that they are stable and soluble in DMF and water. A proton relaxation study on complex 1 has revealed. a relaxivity of 9.8 mM(-1) s(-1) (20 degrees C and 30 MHz), a value that is comparable to those of clinically useful MRI contrast enhancement agents. Complex 1 crystallizes in the triclinic space group P (1) over bar, with a = 12.657(3) Angstrom, b = 14.833(3) Angstrom, c 17.707(3) Angstrom, alpha = 79.65(2)degrees, beta = 86.06(2)degrees, gamma = 68.69(2)degrees, V = 3046.6(12) Angstrom, and Z = 2 (R1 = 0.0534, wR2 = 0.1289). Complex 2 crystallizes in the monoclinic space group P2(1)/n, with a 16.319(2) Angstrom, b = 21.863(2) Angstrom, c 18.410(3) Angstrom, beta 96.85(1)0, V = 6522(2) A(3), and Z = 4 (R1 = 0.0463, wR2 = 0.0750). Complex 3 crystallizes in the triclinic space group P (1) over bar, with a = 11. 173(6) Angstrom, b = 11.534(6) Angstrom, c = 13.311(5) Angstrom, alpha 93.81(3)degrees, beta = 94.82(4)degrees, gamma = 107.20(4)degrees= 1625(2) Angstrom(3), and Z = 2 (R1 0.2979).