The synthesis, X-ray crystal structures, EPR, and magnetic properties of the salts formulated [Fe-III(C(5)Me(5))(2)](4)(POM)(solv)(n), with POM = [(SiMo12O40)-O-VI](4-) (1), [(SiW12O40)-O-VI](4-) (2), [(PMoMo11O40)-Mo-V-O-VI](4-) (3), and [HFeW12O40](4-) (4) and solv = H2O, C3H7ON (DMF), or CH3CN, are reported. Salts 1-3 are isostructural and crystallize in the triclinic space group P $$($) over bar 1. Crystal data for 1 : a = 14.247(11) Angstrom, b = 15.004(4) Angstrom, c = 15.418(2) Angstrom, alpha = 63.26(2)degrees, beta = 83.41(3)degrees, gamma = 69.77(4)degrees, V = 2758(2) Angstrom(3), Z = 1, R = 0.062 for 4662 observed reflections with I greater than or equal to 6 sigma(I). Salt 4 crystallizes in the monoclinic space group C2/c, with a = 29. 495(9) Angstrom, b = 12.545(4) Angstrom, c = 29.482(4) Angstrom, beta = 94.39(4)degrees, V = 10877(10) Angstrom(3), Z = 4, and R = 0.045 for 5305 observed reflections with I greater than or equal to 6 sigma(I). In salts 1-3, the [Fe(C(5)Me(5))(2)](+) cations form linear chains which surround columns of the POM. Salt 4 presents a 3-D structure where the [Fe(C(5)Me(5))(2)](+) occupy the vertices of a cuboctahedron. The 3-D structure results from the association of these cuboctahedrons by sharing vertices in the a and c directions and faces in the b direction. The magnetic properties of these compounds between 2-300 It indicate that the four salts present very weak magnetic interactions obeying the Curie-Weiss expression chi C/(T - Theta) with small values for Theta (1, C = 3.118 emu.mol(-1), Theta = -1.9 K, mu(eff) = 2.497 mu(B); 2, C = 3.113 emu.mol(-1), Theta = -4.5 K, mu(eff) = 2.495 mu(B); 3, C = 3.727 emu.mol(-1), Theta = -3.2 K, mu(eff) = 2.556 mu(B) for the organic radicals and mu(eff) = 1.916 mu(B) for the inorganic anions; 4, C = 6.887 emu.mol(-1), Theta = -0.2 K, mu(eff) = 2.295 mu(B) for the organic radicals and mu(eff) = 5.834 mu(B) for the inorganic anions). The low T magnetization data as well as the EPR spectra also support the absence of magnetic exchange interactions between the two parts. Thus, for compound 4 the magnetization curve at 2 K can be very well reproduced from the sum of contributions of the Fe-III-containing Keggin anion (with S = 5/2 and g = 1.992) and the four [Fe(C(5)Me(5))(2)](+) radicals (with S = 1/2 and g = 2.539). In the EPR spectra the typical signals of both ionic fragments are observed.