Abstract
Five Co^II^ silicotungstate complexes are reported. The centrosymmetric heptanuclear compound K~20~[{(B‐β‐SiW~9~O~33~(OH))(β‐SiW~8~O~29~(OH)~2~)Co~3~(H~2~O)}~2~Co(H~2~O)~2~]⋅47 H~2~O (1) consists of two {(B‐β‐SiW~9~O~33~(OH))(β‐SiW~8~O~29~(OH)~2~)Co~3~(H~2~O)} units connected by a {CoO~4~(H~2~O)~2~} group. In the chiral species K~7~[Co~1.5~(H~2~O)~7~)][(γ‐SiW~10~O~36~)(β‐SiW~8~O~30~(OH))Co~4~(OH)(H~2~O)~7~]⋅36 H~2~O (2), a {γ‐SiW~10~O~36~} and a {β‐SiW~8~O~30~(OH)} unit enclose a mononuclear {CoO~4~(H~2~O)~2~} group and a {Co~3~O~7~(OH)(H~2~O)~5~} fragment. The two trinuclear Co^II^ clusters present in 1 enclose a μ~4~‐O atom, while in 2 a μ~3~‐OH bridging group connects the three paramagnetic centers of the trinuclear unit, inducing significantly larger Co‐L‐Co (L=μ~4~‐O (1), μ~3~‐OH (2)) bridging angles in 2 (θ~av(Co‐L‐Co)~=99.1°) than in 1 (θ~av(Co‐L‐Co)~=92.8°). Weaker ferromagnetic interactions were found in 2 than in 1, in agreement with larger Co‐L‐Co angles in 2. The electrochemistry of 1 was studied in detail. The two chemically reversible redox couples observed in the positive potential domain were attributed to the redox processes of Co^II^ centers, and indicated that two types of Co^II^ centers in the structure were oxidized in separate waves. Redox activity of the seventh Co^II^ center was not detected. Preliminary experiments indicated that 1 catalyzes the reduction of nitrite and NO. Remarkably, a reversible interaction exists with NO or related species. The hybrid tetranuclear complexes K~5~Na~3~[(A‐α‐SiW~9~O~34~)Co~4~(OH)~3~(CH~3~COO)~3~]⋅18 H~2~O (3) and K~5~Na~3~[(A‐α‐SiW~9~O~34~)Co~4~(OH)(N~3~)~2~(CH~3~COO)~3~]⋅18 H~2~O (4) were characterized: in both, a tetrahedral {Co~4~(L~1~)(L~2~)~2~(CH~3~COO)~3~} (3: L~1~=L~2~=OH; 4: L~1~=OH, L~2~=N~3~) unit capped the [A‐α‐SiW~9~O~34~]^10−^ trivacant polyanion. The octanuclear complex K~8~Na~8~[(A‐α‐SiW~9~O~34~)~2~Co~8~(OH)~6~(H~2~O)~2~(CO~3~)~3~]⋅52 H~2~O (5), containing two {Co~4~O~9~(OH)~3~(H~2~O)} units, was also obtained. Compounds 2, 3, 4, and 5 were less stable than 1, but their partial electrochemical characterization was possible; the electronic effect expected for 3 and 4 was observed.