The synthesis, X-ray crystal structures, and ESR and magnetic properties of the anhydrous salt Na-3[Cr(OH)(6)-Mo6O18]. 6(CH3)(2)SO (CrDMSO) containing the Anderson-Evans polyanion, of two new molecular compounds based on this polyanion ([Fe(C5Me5)(2)](3)[Cr(OH)(6)Mo6O18]. 2OH(2)O [FeCr(1)], Na[Fe(C5H5)(2)](2)[Cr(OH)(6)Mo(6)0(18)]. 3H(2)O [FeCr(2)]), and of another based on a Keggin polyanion, [Fe(C5Me5)(2)](4)[HPCu(H2O)W11O39]. 6CH(3)CN (FeCu), are reported. CrDMSO : monoclinic, C2/c, a = 25.37(2) Angstrom, b = 14.632(2) Angstrom, c = 15.455(6) Angstrom, beta = 123.17(4)degrees, Z = 4. FeCr(1) : triclinic, P (1) over bar, a = 12.489(3) Angstrom, b = 14.113(3) Angstrom, c = 15.662(4) Angstrom, alpha = 101.55(2)degrees, beta = 105.82(2)degrees, gamma = 110.23(2)degrees, Z = 1. FeCr(2) : monoclinic, P2(1)/n, a = 10.015(2) Angstrom, b = 7.722(6) Angstrom, c = 25.137(5) Angstrom, beta = 90.626(9)degrees, Z = 2. PeCu : monoclinic, P2(1)/n, a = 15.596(7) Angstrom, b = 15.138(3) Angstrom, c = 25.921(12) Angstrom, beta = 97.53(2)degrees, Z = 2. In the compound FeCr(1) the Fe(C5Me5)(2) units are perpendicular to each other and form a centrosymmetrical group of three cations, which alternates with the planar [Cr(OH)(6)Mo6O18](3-) anion to form perpendicular mixed chains along the [21(1) over bar] and [(2) over bar 1 (1) over bar] directions. In the compound FeCu, the decamethylferrocenium units form a polyhedral structure which encapsulates the Keggin polyanion. The magnetic propel-ties indicate that the salts containing metallocenium radicals present weak antiferromagnetic interactions obeying the Curie-Weiss expression chi = Cl(T - Theta) with small negative values for Theta (FeCr(1), Theta = -0.5, -0.4; PeCr(2), Theta = -0.6; FeCu, Theta = -0.8). The ESR spectra for compounds containing chromium atoms may reveal a zero field splitting of the central chromium atom of the Anderson-Evans polyanions, which is directly related to the local symmetry of the chromium in its octahedral site.