The isomorphic series of double sulfites with empirical formula Cu2SO3.MSO3.2H 2O (where M is Cu, Fe, Mn, or Cd) have been prepared from the Cu(II) replacement by transition metal ions such as Mn(II), Fe(II) and Cd(II) ions in Chevreul's salt, Cu2SO3.CuSO3.2H 2O. As a consequence, the isomorphic species present distinct colors. Molecular modeling calculations were carried out for the dimeric [CuI2(SO3) 2(SO3)2]6- center. The electronic spectra of the Chevreul's salt consist of a charge-transfer band around 425 nm associated with the [CuI2(SO3) 2(SO3)2]6- chromophore and two ligand field transitions at 785 and 1000 nm involving the Jahn-Teller splitting of the Cu(II) levels. An additional intervalence-transfer band, responsible for its characteristic red color, can be found at 500 nm. The replacement of the Cu(II) ions for Fe(II), Mn(II) and Cd(II) does not eliminate the absorption band at 425 nm, supporting its assignment as a charge-transfer transition centered on the Cu(I) sites; while the original band at 500 nm disappears, in agreement with its intervalence transfer nature.
electronic spectra; Chevreul's salt; isomorphic salts
