Optical rotatory power of co-ordination compounds. Part XIV. Crystal spectrum and circular dichroism of α-Ni(H2O)6SO4

Abstract

The axial and ortho-axial crystal spectra and the c.d. of α-Ni(H₂O)₆SO₄ have been measured at room and at liquid nitrogen temperatures. The results indicate that neither first-order spin–orbit coupling nor trigonal, tetragonal, or orthorhombic distortions significantly affect the accessible electronic states of the quasi-octahedral [Ni(H₂O)₆]²⁺ ion in the crystal. The lowest-energy ligand-field transition to the ³T_2g excited-state is found to be magnetic-dipole allowed, and the higher-frequency red band is ascribed to two transitions resulting from the second-order spin–orbit coupling of the ¹E_g(D) and ³T_1g(F) states which are accidentally degenerate in the zero-order. The latter assignment is supported by an analysis of the vibronic structure observed in the red band, particularly in the low-temperature c.d.