Irreversible spectral changes of cobalt(II) by moderate heating in sol-gel glasses, and their ligand field rationalization

Cobalt(II) nitrate or acetate added to a precursor methanolic solution of TMOS (silicon tetrakis-methoxide), which hydrolyzes and polycondensates when heated above 300°C, forms blue films showing three strong bands at 15600, 16800 and 19000 cm^-1 (like a known complex with two acetone and two chloro ligands). The absorption spectra even of tetrahalides and crystalline oxides (e.g. spinel-type) do not agree closely with regular tetrahedral coordination, and angular overlap estimates suggest D_2h holohedrized symmetry in the glass. Inner shell 3p mixing with 3d may provide intensities not feasible in octahedral complexes.