Structural conversion of [Fe4S4] clusters into [Fe2S2] clusters has been suggested to be a fundamental process for some O-2-sensing proteins. While the formation of [Fe2S2] clusters from synthetic [Fe4S4] clusters has been unprecedented, an all-ferric [Fe4S4](4+) cluster Fe4S4{N(SiMe3)(2)}(4) (1) was found to split in the presence of pyridines, giving [Fe2S2] clusters Fe2S2{N(SiMe3)(2)}(2)(L)(2) (2, L = pyridines). The structural conversion between [Fe4S4] and [Fe2S2] clusters appeared to be reversible, and the thermodynamic parameters for the equilibrium reactions between 1 + L and 2 were determined. Assembly of two [Fe2S2] clusters was also induced by chemical reductions of Fe2S2{N(SiMe3)(2)}(2)(Py)(2) (Py = pyridine), and the resultant [Fe4S4] clusters [1](-) and [1](2-) were found to split into two [Fe2S2] clusters by oxidation with [Cp2Fe](+) in the presence of pyridine.