[(CH 3 ) 4 N] 2 FeGe 4 S 10 and Cs 2 FeGe 4 S 10 , A Solid-State Example of Molecular Orbital Theory

[(CH3)4N]2FeGe4S10 and Cs2FeGe4S10 are a pair of isostructural open-framework materials differing only in the charge balancing species occupying the void spaces of the three-dimensional FeGe4S102- lattice. These materials have sufficient local flexibility to distort around the void space cations to minimize the energy of the entire structure. The result is two materials contg. S4 symmetry site Fe2+ bonded to four chem. identical sulfur atoms. The bond lengths for the sulfur are essentially identical between the two phases but with significantly different bond angles. As a result, the near-IR d-d spectroscopy of these materials is qual. different. These differences may be readily explained by a simple model using MO theory