Other fixatives used for electron microscopy are osmium tetroxide or uranyl acetate. The hydroxide dissolves in strongly alkaline solution to give hydroxo complexes of the uranyl ion. It was dissolved in nitric acid to produce uranyl nitrate. The nature of uranyl nitrate complexes with trialkyl phosphates have been characterized. Most procedures in electron microscopy for biology require the use of uranyl acetate. Incorporated uranium becomes uranyl ions, which accumulate in bone, liver, kidney, and reproductive tissues. The neptunyl and uranyl ions can also form a complex together. The uranyl ion is always associated with other ligands. The uranyl fluoride solid acts to plug the leak, limiting further escape of depleted UF. The colour of uranyl compounds is due to ligand-to-metal charge transfer transitions at ca. It is easier to observe in compounds of the heavier elements, such as uranyl compounds. Following protein engineering, however, it has also been able to participate in uranyl ion reduction. The uranyl ion, , has a "trans" structure. Uranyl nitrate is important for nuclear reprocessing. Uranyl acetate is both radioactive and toxic.