Perplexing EPR Signals from 5f³6d¹ U(II) Complexes

Metal complexes with unpaired electrons in orbitals of different angular momentum quantum numbers (e.g., 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 5f(3)6d(1) electron configurations and on some analogous Ce(II), Pr(II), and Nd(II) complexes with 4f(n)5d(1) electron configurations. The U(II) compounds unexpectedly display similar two-line axial signals with g(||)= 2.04 and g(perpendicular to) = 2.00 at 77 K. In contrast, U(II) complexes with 5f(4) configurations are EPR-silent. Unlike U(II), the congenic 4f(3)5d(1) Nd(II) complex is EPR-silent. The Ce(II) complex with a 4f(1)5d(1) configuration is also EPR-silent, but a signal is observed for the Pr(II) complex, which has a 4f(2)5d(1) 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 5f(3)6d(1) configuration, the results are viewed as unexplained phenomena. However, they do show that 5f(3)6d(1) U(II) samples can be differentiated from 5f(4) U(II) complexes by EPR spectroscopy.