Electronic properties of transition-metal complex ions adsorbed on silica gel. Part II. Cobalt(II) and cobalt(III)

Abstract

The electronic spectra and magnetic moments of a series of complexes of the cobalt(II) and cobalt(III) ions, adsorbed on silica gel by an exchange process, have been examined. The Co(H₂O)₆ ²⁺ cation reacts with the hydrated surface to form a blue tetrahedral species which probably involves a CoO₃′O₁″ chromophore on the surface, but a pink octahedral species is formed when the hydrated gel is allowed to stand. With nitrogen chelate ligands (en and dien) partial reaction occurs on the hydrated gel depending on the ratio of ligand to cobalt(II) ion; more extensive reaction occurs upon dehydration. The electronic properties suggest the formation of four-, five-, and six-co-ordinate species depending on the conditions. In the presence of air, extensive oxidation to cobalt(III) occurs. These results suggest that the cobalt(II) ion reacts much more strongly with the gel surface than does the nickel(II) ion. With cobalt(III) complexes no reaction occurs with the hydrated gel, but dehydration results in the reduction of the cobalt(III) complexes to the corresponding cobalt(II) surface species. Possible mechanisms for this reaction are discussed.