SPECTROSCOPIC STUDIES OF EXCHANGE INTERACTIONS IN POLYNUCLEAR TRANSITION METAL COMPLEXES II. [(NH3)5Cr(OH)Cr(NH3)5] Cl5,3H20.

Abstract

Single crystals of phydroxo-bis[pentaamminechromium(III)] chloride monohydrate have been prepared. The spectroscopic properties of these crystals have been measured and two crystal forms have been characterized. The first form is obtained from solutions at 2�C and it changes to the second, stable form, at room temperature. The latter is tetragonal, space group D44-P41212 with a = 16.28 %L, c = 14.84 A, and Z = 8. The spectroscopic properties require two inequivalent sets of dimer units in the unit cell and this result is consistent with the X-ray data. The spectroscopic properties of the first form require four inequivalent sets of dimers and the ground state energy levels of each of these are described by the isotropic Hamiltonian H = JSa . Sb - j(Sa. Sb)2 with the following values: set 1, J = 30.4+/- 0.2, j = 0.32+/-0. 10; set 2, J = 30.4 +/-0.2, j = 0.221 0.10; set 3, J = 31.4 +/-0.2, j = 0-18+/- 0.10; set 4, J = 31-4+/- 0.4, j = 0.0+/- 0.20. High-resolution luminescence excitation spectroscopy shows that sets 1 and 2 are not strongly coupled to sets 3 and 4. The small differences between sets 1 and 2 and between sets 3 and 4 suggest that these differences arise from structural disorder within the two inequivalent sets of the room temperature stable form. Previous differences in the interpretation of the earlier magnetic and