| Summary: | Five new complexes with metal cations (Mn<sup>2+</sup>, Fe<sup>3+</sup>, Co<sup>2+</sup>, Ni<sup>2+</sup>, and Cu<sup>2+</sup>) of monolacunary Keggin monovanado-deca-tungstophosphate, K<sub>8</sub>[PVW<sub>10</sub>O<sub>39</sub>]·15H<sub>2</sub>O, have been synthesised. The molar ratio of the combination between metal cations and K<sub>8</sub>[PVW<sub>10</sub>O<sub>39</sub>]·15H<sub>2</sub>O has been established to be 1:1, and its general molecular formulas were found to be: K<sub>n</sub>[MPVW<sub>10</sub>O<sub>39</sub>(H<sub>2</sub>O)]·xH<sub>2</sub>O (<i>n</i> = 5 for M = Fe<sup>3+</sup> and <i>n</i> = 6 for M = Mn<sup>2+</sup>, Co<sup>2+</sup>, Ni<sup>2+</sup>, and Cu<sup>2+</sup>). Optimal conditions for complexes’ synthesis (pH, temperature, and reaction time) have been determined. The characterisation of K<sub>8</sub>[PVW<sub>10</sub>O<sub>39</sub>]·15H<sub>2</sub>O and of its compounds K<sub>n</sub>[MPVW<sub>10</sub>O<sub>39</sub>(H<sub>2</sub>O)]·xH<sub>2</sub>O have been performed using AAS, TG-DTA-DTG, UV-VIS, IR, Raman, and powder XRD methods. In UV spectra, two maximums of absorption were obtained, at 200 and 250 nm, characteristic of Keggin polycondensate compounds. The coordination of cations Ni<sup>2+</sup>, Co<sup>2+</sup>, and Cu<sup>2+</sup> through oxygen atoms from K<sub>8</sub>[PVW<sub>10</sub>O<sub>39</sub>]·15H<sub>2</sub>O in an octahedron system has been reflected with VIS spectroscopy. All these methods have proved the compositions and structures of K<sub>8</sub>[PVW<sub>10</sub>O<sub>39</sub>]·15H<sub>2</sub>O and K<sub>n</sub>[MPVW<sub>10</sub>O<sub>39</sub>(H<sub>2</sub>O)]·xH<sub>2</sub>O, their similarity with other vanadotungstophosphates, and their achievements in the Keggin class. Additionally, all analysis methods have shown an increase in the degree of structural symmetry and the thermal stability of a polyoxoanion complex after attaching metal cations compared to the monolacunary, K<sub>8</sub>[PVW<sub>10</sub>O<sub>39</sub>]·15H<sub>2</sub>O.
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