Russell-Saunders coupling, J = |L – S|, ..., L + S, i.e., maximum spin-orbit coupling (SOC), is the dominant factor in the electronic structure of the f block ions. For the d block ions, this coupling is typically considered only in their free-ions - atomic spectroscopy. In molecular complexes of the d block ions, the effects of orbital angular momentum are usually ignored, or handled by a spin Hamiltonian model in which the effects of SOC are treated by a spin-only zero-field splitting (zfs), usually given by a tensor that can be represented by a uniaxial component D and a rhombic component E (higher order terms are also possible) and by anisotropy in the g matrix. This approach is perfectly reasonable when the electronic ground state is orbitally non-degenerate, such for nickelocene (NiCp2), which has a 3A2 ground state. We will present frequency-domain Fourier-transform THz electron paramagnetic resonance (FD-FT THz-EPR) data for NiCp2 that demonstrates the successful use of a spin Hamiltonian for S = 1, giving D = 31 cm-1, along with an analysis using ligand-field theory and DFT calculations, as well as a comparison with other experimental techniques, such as magnetometry. Experimental results for decamethylnickelocene (NiCp*2) will also be presented. In other high symmetry molecules where the orbital angular momentum is unquenched (i.e., an orbitally degenerate ground state), then the spin Hamiltonian analysis is not as appropriate, and a complete ligand-field treatment is preferable. We will demonstrate this by presenting high-frequency and -field EPR (HFEPR) studies of chromocene (CrCp2), for which the ground state is likely 3E. We will also discuss results from our own work and that of others where the application of a spin Hamiltonian may be improved upon by use of a more complete treatment. These examples include complexes with unquenched angular momentum, such as in tetragonal Ti(II), trigonal Fe(II), trigonal Ni(II), and 8-coordinate Co(II).

Spin-orbit coupling: Not just for f elements

249th National Meeting (2015)

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