A thermal spin transition in [Co(bpy)(3)][LiCr(ox)(3)] (ox = C2O42-; bpy =2,2 '-bipyridine)

In the three-dimensional oxalate network structures [M-II(bpy)(3)][(MMIII)- M-I(ox)(3)] (ox = C2O42-; bpy=2,2'-bi-pyridine) the negatively charged oxal ate backbone provides perfect cavities for tris-bipyridyl complex cations. The size of the cavity can be adjusted by variation of the metal ions of th e oxalate backbone. In [Co(bpy)(3)] [NaCr(ox)(3)], the [Co(bpy)(3)]2+ compl ex is in its usual T-4(1)(t(2g)(5e)g(2)) high-spin ground state. Substituti ng Na+ by Li+ reduces the size of the cavity. The resulting chemical pressu re destabilises the high-spin state: of [Co(bpy)(3)](2+) to such an extent that the E-2(t(2g)(6)e(g)(1)) low-spin state becomes the actual ground stat e. As a result, [Co(bpy)(3)] [LiCr(ox)(3)] becomes a spin-crossover system, as shown by temperature-dependent magnetic susceptibility measurements and single-crystal optical spectroscopy, as well as by an X-ray structure dete rmination at 290 and 10 K.