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 EPR data were collected at <10 K and 77 K on ten U(II) complexes with 5f36d1 electron configurations and on Ce(II), Pr(II), and Nd(II) complexes with 4fn5d1 electron configurations. The U(II) compounds unexpectedly display similar two-line axial signals with g|| = 2.04 and g⊥ = 2.00 at 77 K and U(III)-like spectra at 5 K. In contrast, U(II) complexes with a 5f4 configuration are EPR silent. Unlike U(II), the congener Nd(II) complex is EPR-silent although it possesses an analogous 4f35d1 configuration. The Ce(II) complex with a 4f15d1 configuration is also EPR silent, but a signal is observed for the Pr(II) complex, which has a 4f25d1 configuration. Whether or not an EPR signal is expected for these complexes depends on the coupling between f and d electrons. If the coupling is sufficiently strong, no signal is expected for ions with an even number of electrons, which is the observation for the lanthanide species. If the coupling is very weak, separate EPR signals for the f and d systems might be observed even in systems with an even number of electrons. The Coulomb interaction between 5f and 6d electrons is strong in trivalent uranium systems, which results in strong coupling between the 5f and 6d electrons. This coupling will be weaker in divalent uranium systems, but it is still expected to be sufficiently strong to preclude an EPR signal from compounds with a 5f36d1 configuration. Some potential sources of the EPR signal from the U(II) complexes are discussed.