Synthesis, crystal structure and electrochemical behavior of tetranuclear transition metal clusters based on lacunary silicotungstates:: [M4(H2O)2(SiW9O34)2]10- (M = Ni2+, Co2+) and [Fe4(μ-O)2(μ-OH)2(SiW10O37)2]14-

Three tetranuclear transition metal clusters based on lacunary silicotungstates [M-4(H2O)(2)(SiW9O34)(2)](12-) (M = Ni2+(1), Co2+(2)), and [Fe-4(mu-O)(2)(mu-OH)(2)(SiW10 O-37)(2)](14-) (3) have been synthesized under ambient conditions and characterized by elemental analyses, IR, TG, cyclic voltammetry, and single-crystal X-ray diffraction. The polyoxoanions of 1 and 2 are isostructural, including a central rhomb-like {M4O16} (M = Ni, Co) cluster sandwiched by two trivacant {B-alpha-SiW9} Keggin moieties. In the polyoxoanion of 3, two mu-OH and two mu-O bridges link with four Fe-III ions, forming an eight-membered ring. This [Fe-4(mu-OH)(2) (mu-O)(2)] aggregation is sandwiched by two bi-vacant {alpha-SiW10} Keggin fragments. The electrochemical properties of the three compounds were investigated.