EPR studies of paramagnetic rhodium centers in LiH and LiD

EPR spectra have been obtained for two independent paramagnetic Rh centers in UV-irradiated single crystals of LiH and LiD. The spin Hamiltonians for both species indicate S=1/2 and are assigned to Rh2+ ions with low-spin 4d7 electron configurations. Analysis of the anisotropic g-factor for the tetragonal center shows the unpaired electron is the Rh 4dx2−y2 orbital. The unpaired electron occupies the 4d3z 2−r 2 orbital in the orthorhombic center. The g factor, hyperfine constants, and protium superhyperfine constants for the Rh centers in LiH and LiD are compared with similar parameters for Rh2+ ions in several halide and oxide crystals. Although the isotropic protium hyperfine values indicate significant ‖1s≳−‖4d≳ orbital interactions, the total covalency in the Rh–H(D) bonds is rather small since the hydrogen ‖2p≳ orbitals cannot participate. The displacements of H ligands in the Rh centers have been determined from the direct-dipolar contributions to the superhyperfine tensor.