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A new series of sodium/potassium salts of sandwich-type [Mn+₃(H₂O)_x(BiW₉O₃₃)₂](18 − 3n)− heteropolyanions (Mn+  = (VO)II, x  = 0 (1) and Mn+  = CrIII (2), MnII (3), FeIII (4), CoII (5), NiII (6), CuII (7), x  = 3) were synthesized and characterized by means of elemental analyses, thermogravimetry, FT-IR, UV-VIS, EPR and HF-EPR spectroscopy. FT-IR data are proof of the heteropolyanion frame formation and indicate the co-ordination of each transition metal at two corner-shared octahedra from the trivacant Keggin fragments. The low shift of the ν_asym(Bi–O_a) band in the spectra of the CuII and FeIII compounds relative to the ligand spectrum suggests a sterically-inert s2 lone pair on each BiIII ion. The charge transfer band p_π(O_t)→d_π*(W) at 193.6 nm is not affected by the co-ordination of the metals, but the p_π(O_c,e)→d_π*(W) band shifts towards higher energies for CrIII and FeIII complexes or lower energies for the other complexes. d–d transitions obtained in the VIS spectra of the complexes have been interpreted in terms of five-co-ordination of every transition metal and a distorted square-pyramidal local symmetry. EPR and HF-EPR data show the presence of small antiferromagnetic interactions in the spin frustrated trinuclear metallic clusters (J  =   −4.116 cm−1 for the CrIII complex, J  =   −2.074 cm−1 for the MnII complex). Two species with different degrees of rhombical distortion have been identified in the case of the FeIII (D₁  = 1.5 cm−1, E₁  = 0.12 cm−1 and D₂  = 1.5 cm−1, E₂  = 0.5 cm−1) and CoII (g_x1  = 5.021, g_y1  = 3.561, g_z1  = 2.401 and g_x2  = 5.776, g_y2  = 3.813, g_z2  = 2.850) samples.