Thorium(IV) nitrate

Thorium(IV) nitrate is a chemical compound, a salt of thorium and nitric acid with the formula Th(NO₃)₄. A white solid in its anhydrous form, it can form tetra- and pentahydrates. As a salt of thorium it is weakly radioactive.

Thorium(IV) nitrate hydrate can be prepared by the reaction of thorium(IV) hydroxide and nitric acid:

Different hydrates are produced by crystallizing in different conditions. When a solution is very dilute, the nitrate is hydrolysed. Although various hydrates have been reported over the years, and some suppliers even claim to stock them, only the tetrahydrate and pentahydrate actually exist. What is called a hexahydrate, crystallized from a neutral solution, is probably a basic salt.

The pentahydrate is the most common form. It is crystallized from dilute nitric acid solution.

The tetrahydrate, Th(NO₃)₄·4H₂O is formed by crystallizing from a stronger nitric acid solution. Concentrations of nitric acid from 4 to 59% result in the tetrahydrate forming. The thorium atom has 12-coordination, with four bidentate nitrate groups and four water molecules attached to each thorium atom.

To obtain the anhydrous thorium(IV) nitrate, thermal decomposition of Th(NO₃)₄·2N₂O₅ is required. The decomposition occurs at 150–160 °C (302–320 °F).

Anhydrous thorium nitrate is a white substance. It is covalently bound with low melting point of 55 °C (131 °F).

The pentahydrate Th(NO₃)₄·5H₂O crystallizes with clear colourless crystals in the orthorhombic system. Each thorium atom is connected twice to each of four bidentate nitrate groups, and to three water molecules via their oxygen atoms. In total the thorium is eleven-coordinated. There are also two other water molecules in the crystal structure.The water is hydrogen bonded to other water, or to nitrate groups.