Perplexing EPR Signals from 5f6d U(II) Complexes.

Metal complexes with unpaired electrons in orbitals of different angular momentum quantum numbers (., f and d orbitals) are unusual and opportunities to study the interactions among these electrons are rare. X-band electron paramagnetic resonance (EPR) data were collected at <10 and 77 K on 10 U(II) complexes with 5f6d electron configurations and on some analogous Ce(II), Pr(II), and Nd(II) complexes with 4f5d electron configurations. The U(II) compounds unexpectedly display similar two-line axial signals with 2.04 and = 2.00 at 77 K. In contrast, U(II) complexes with 5f configurations are EPR-silent. Unlike U(II), the congenic 4f5d Nd(II) complex is EPR-silent. The Ce(II) complex with a 4f5d configuration is also EPR-silent, but a signal is observed for the Pr(II) complex, which has a 4f5d configuration. Whether or not an EPR signal is expected for these complexes depends on the coupling between f and d electrons. Since the coupling in U(II) systems is expected to be sufficiently strong to preclude an EPR signal from compounds with a 5f6d configuration, the results are viewed as unexplained phenomena. However, they do show that 5f6d U(II) samples can be differentiated from 5f U(II) complexes by EPR spectroscopy.