Abstract. The K₉[PV₃W₉O₄₀].19H₂O complex was synthesized and investigated by FT-IR, UV-VIS and ESR spectroscopy. The appearance of v_as(V=O) ≈ 989 cm^-1 vibration in the FT-IR spectrum of the complex and the shift of the v_as(W=O_d) and v_as(W-O_bc-W) vibrational bands indicate the coordination of the vanadium ions to the trilacunary Keggin unit. The three transitions in the visible electronic spectrum of the complex were assigned to ²B₂ → ²E (14185 cm^-1), ²B₂ → ²B₁ (19170 cm^-1) for V(IV) ions in C_4v local symmetry and to the V(IV) → W(VI) (24390 cm^-1) charge transfer. The isotropic ESR spectrum of the complex, simulated by considering a Gaussian lineshape and g₀ = 2.009, ΔB_pp = 265 G parameters, indicates the presence of dipolar interactions between the V(IV) ions.

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