VOID GEOMETRY DRIVEN SPIN-CROSSOVER IN ZEOLITE-ENCAPSULATED COBALT TRIS(BIPYRIDYL) COMPLEX ION

The cobalt(II) tris(bipyridyl) complex ion encapsulated in zeolite-Y s upercages exhibits a thermally driven interconversion between a low-sp in and a high-spin state-a phenomenon not observed for this ion either in solid state or in solution. From a comparative study of the magnet ism and optical spectroscopy of the encapsulated and unencapsulated co mplex ion, supported by molecular modeling, such spin behavior is show n to be intramolecular in origin. In the unencapsulated or free state, the [Co(bipy)(3)](2+) ion exhibits a marked trigonal prismatic distor tion, but on encapsulation, the topology of the supercage forces it to adopt a near-octahedral geometry. An analysis using the angular overl ap ligand field model with spectroscopically derived parameters shows that the geometry does indeed give rise to a low-spin ground state, an d suggests a possible scenario for the spin state interconversion.