Spectroscopic and electron paramagnetic resonance behavior of trinuclear metallic clusters encapsulated in [Mn+<SUB>3</SUB>(H<SUB>2</SUB>O)<SUB>x</SUB>(BiW<SUB>9</SUB>O<SUB>33</SUB>)<SUB>2</SUB>](18−3n)− heteropolyanion (Mn+  = (VO)II, x  = 0 and Mn+  = CrIII, MnII, FeIII, CoII, NiII, CuII, x  = 3)

A new series of sodium/potassium salts of sandwich-type [Mⁿ⁺₃(H₂O)_x(BiW₉O₃₃)₂]^{(18 − 3n)−} heteropolyanions (Mⁿ⁺  = (VO)^II, x  = 0 (<compoundref idrefs="chem1">1</compoundref>) and Mⁿ⁺  = Cr^III (<compoundref idrefs="chem2">2</compoundref>), Mn^II (<compoundref idrefs="chem3">3</compoundref>), Fe^III (<compoundref idrefs="chem4">4</compoundref>), Co^II (<compoundref idrefs="chem5">5</compoundref>), Ni^II (<compoundref idrefs="chem6">6</compoundref>), Cu^II (<compoundref idrefs="chem7">7</compoundref>), 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 Cu^II and Fe^III compounds relative to the ligand spectrum suggests a sterically-inert s² lone pair on each Bi^III 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 Cr^III and Fe^III 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 Cr^III complex, J  =   −2.074 cm⁻¹ for the Mn^II complex). Two species with different degrees of rhombical distortion have been identified in the case of the Fe^III (D₁  = 1.5 cm⁻¹, E₁  = 0.12 cm⁻¹ and D₂  = 1.5 cm⁻¹, E₂  = 0.5 cm⁻¹) and Co^II (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.