LIGAND-FIELD EFFECTS IN THE HYDRATED DIVALENT AND TRIVALENT METAL-IONS OF THE FIRST AND 2ND TRANSITION PERIODS

The influence of d-shell occupation on the structure of the first hydr ation sphere for a series of hydrated transition metal ions has been e valuated by using theoretical calculations to optimize the geometry an d find the conformations with lowest energy. General trends in the pro perties reflecting the calculated metal-oxygen bond strength and energ y are discussed and compared with experimental values. The metal-oxyge n distances, calculated by ab initio SCF methods using large basis set s for high-spin hexaaqua complexes in the T-h symmetry of the di- and trivalent 3d metal ions, follow closely the trends found in crystal st ructure determinations of the isomorphous series of the 3d hexaaqua me tal ions in Tutton and alum salts. The variation of the binding energi es in a hexaaqua complex shows the double-humped features expected for a splitting of the d orbitals in an octahedral ligand field with the largest stabilization at formal d(3) and d(8) electron configurations of the metal ions. In cases with degenerate d-orbitals, an additional splitting of the energy levels by a lowering of the symmetry of the he xahydrated ion will in some cases significantly increase the binding e nergy. For the largest d(1) and d(6) metal ions an ''all-vertical'', a nd for some large d(2) ions an ''air-horizontal'', conformation of the hydrogen atoms of the planarly coordinated water ligands around the t rifold axis in the D-3d symmetry is favored. For the hexahydrated d(9) ion Ag2+, the first-order Jahn-Teller effect leading to a tetragonal elongation of the octahedral coordination has been studied. The possib ility that the nondistorted d(4) [Cr(H2O)(6)](2+) complex is forced in to a low-spin state in its hexafluorosilicate salt due to compression in the lattice has been considered. The electrostatically dominated bi nding in the hexaaqua complexes shows an increasing covalent contribut ion to the right in the transition rows, especially for the trivalent ions. The ligand field effects are generally larger for the trivalent than for the divalent ions and also larger for the 4d(n) than for the 3d(n) elements, although the overall bond strength is lower due to the larger size of the 4d(n) ions. The high ligand field strength results in low-spin ground states for the hexahydrated Co3+, RU(2+), Rh3+ (d( 6)), and RU(3+) (d(5)) ions in solution but, except for Rh3+, these hi gh- to low-spin transitions proved to be difficult to reproduce by com putations for the isolated hexaaqua clusters. Electron correlation was introduced, but the effect of the surroundings, which also may provid e an additional important contribution to the stabilization of the low -spin state, was not accounted for. The low-spin square-planar configu rations of the d(8) complexes [Pd(H2O)(4)](2+) and [Ag(H2O)(4)](3+) ar e discussed in terms of ligand field effects.