Structural and vibrational properties of U(VI)O2Cl4(2-) and Pu(VI)O2Cl4(2-) complexes

In actinide chemistry, it has been shown that equatorial ligands bound to the metal centers of actinyl ions have a strong influence on the chemistry and therefore the electronic structure of the O═An═O moiety. While this influence has received a significant amount of attention, considerably less research has been done to investigate how the identity of the actinide metal itself (U, Np, Pu, Am) affects the actinyl stretching frequencies. Herein, we present the structural and spectroscopic characterization of six actinyl tetrachloride compounds (M2AnO2Cl4: M = Rb, Cs, Me4N; An = U, Pu) as well as the stretching and interactive force constants of the actinyl moiety in each species. Our results show a decrease in the stretching force constant and a weakening of the An═O bond when traversing the actinides from uranyl to plutonyl, which is interesting because the solid state molecular structures show a slight contraction of the An═O bond length when uranium is replaced with plutonium. Additionally, the interaction force constants for both the uranyl and plutonyl compounds were found to be negative, which corresponds to a reduction of the force constant for the symmetric stretching mode.