Electronic Structure, Spectra, and Magnetic Properties of Oxycations. III. Ligation Effects on the Infrared Spectrum of the Uranyl Ion

Abstract

A series of uranyl complexes K_xUO₂L_y(NO₃)₂, where L is the variable ligand (or ligands), has been prepared; it has been shown that a ligand series may be defined using the antisymmetric stretching frequency of the uranyl entity, and this series exhibits a remarkable parallelism with the spectrochemical series defined by Δ in octahedral complexes of transition metals of the 1st and 2nd series. This parallelism has been rationalized using a mixed ligand field theory in which the uranyl ion is considered subject to bonding with ligands which are arranged hexagonally in a plane equatorial to the O–U–O axis. It is shown that the large changes of ν₃ and ν₁ are due primarily to electron population of the φ_u and δ_u atomic orbitals of uranium. Such population is physically equivalent to the reductions AmO₂^{+ +}→AmO₂⁺ and N_pO₂^{+ +}→NpO₂⁺, which cause a decrease of approximately 100 cm^—1 in ν₃. It is further shown that ${\mathrm{\Delta \nu ̄}}_3$ = — electrostatic effect — σ(L→M) — π(L→M) ± π(M→L), where in the last term the plus sign is the more probable, and the results obtained have been rationalized using this equation. Some evidence in favor of linearity of the UO₂^{+ +} has also been induced, and a general criterion for distinguishing the difficulty observable ν₁ has been reestablished. It has also been shown, although not discussed extensively, that the stability of uranyl complexes should increase, roughly as ${\mathrm{\nu ̄}}_3$ (or ${\mathrm{\nu ̄}}_1$ ) decreases. The discussion of similar effects in vanadyl, titanyl, chromyl, and other oxycations has been deferred to a later time.