The colours and magnetic properties of hydrated iron group salts, and evidence for covalent bonding

TLDR: In this paper, the authors analyzed the optical absorption data of paramagnetic salts containing [M(H 2 O)$O)$\6}, M = 3d$n}, and showed that the orbital level separations given by the optical spectrum are smaller than those predicted from magnetic data.

Abstract: The typical colours of paramagnetic salts containing [M(H$\_{2}$O)$\_{6}$], M = 3d$^{n}$, complexes, are generally assumed to arise from optical transitions between the orbital energy levels of the 3d ion which are split by the crystalline electric field due to the surrounding water dipoles. The available optical absorption data are analyzed using this ionic model and the associated crystal field theory, and it is shown that while experiment and theory agree fairly well, there are systematic discrepancies. In addition, the orbital level separations given by the optical spectrum are found to be smaller than those predicted from magnetic data. It is shown how these discrepancies can be accounted for by introducing weak covalent bonds into the [M(H$\_{2}$O)$\_{6}$] complex, so that there is charge transfer between the paramagnetic ion and the attached water molecules.