Sandwich-Type Phosphotungstates:  Structure, Electrochemistry, and Magnetism of the Trinickel-Substituted Polyoxoanion [Ni<sub>3</sub>Na(H<sub>2</sub>O)<sub>2</sub>(PW<sub>9</sub>O<sub>34</sub>)<sub>2</sub>]<sup>11-</sup>

The novel nickel-substituted, dimeric phosphotungstate [Ni<sub>3</sub>Na(H<sub>2</sub>O)<sub>2</sub>(PW<sub>9</sub>O<sub>34</sub>)<sub>2</sub>]<sup>11-</sup> (<b>1</b>) has been synthesized and characterized by IR spectroscopy, elemental analysis, and electrochemistry. X-ray single-crystal analysis was carried out on Na<sub>11</sub>[Ni<sub>3</sub>Na(H<sub>2</sub>O)<sub>2</sub>(PW<sub>9</sub>O<sub>34</sub>)<sub>2</sub>]·21.25H<sub>2</sub>O, which crystallizes in the triclinic system, space group <i>P</i>1̄, with <i>a</i> = 12.2467(6) Å, <i>b</i> = 16.6031(7) Å, <i>c</i> = 22.4017(12) Å, α = 73.9870(10)°, β = 87.6060(10)°, γ = 79.344(2)°, and <i>Z</i> = 2. The polyanion consists of two lacunary <i>B-</i>α<i>-</i>[PW<sub>9</sub>O<sub>34</sub>]<sup>9-</sup> Keggin moieties linked via three nickel(II) centers and a sodium ion. The structure of <b>1</b> is composed of two fused Keggin fragments that represent different Baker−Figgis isomers (α- vs β-type). Electrochemical studies show that <b>1</b> exhibits a stable and reproducible voltammetric pattern, with a first wave featuring a chemically reversible four-electron/four-proton process. An investigation of the magnetic properties indicates that the three nickel centers exhibit ferromagnetic exchange interaction.