The coordination environment of a molecular UO_2~(2+) ion in aqueous solution is determined with X-ray scattering and interpreted taking into account the electron redistribution around the uranyl ion. The data indicate that uranyl coordinated to five water molecules is the dominating species, although a small percentage of the uranyl ions are coordinated to four waters. We argue that our result, taken together with calculated relative stabilities of 4- and 5-coordinated UO_2~(2+), are consistent with a dynamic equilibrium that favors five coordinating H_2O. The data further indicate that electron transfer from the uranium(VI) to the uranyl-oxygen is incomplete and that the effective charge of the uranium is less than indicated by its formal valence. In addition, a partial electron transfer from the hydrating waters to the uranium takes place, further lowering the effective charge of the uranium atom. The results described herein are in agreement with recent density functional calculations for an actinyl-water cluster embedded in a dielectric continuum.
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