Reinecke's salt

Reinecke's salt is a chemical compound with the formula NH4[Cr(NCS)4(NH3)2].H2O. The dark-red crystalline compound is soluble in boiling water and ethanol.

Structure
The chromium atom is surrounded by six nitrogen atoms in an octahedral geometry; the NH3 ligands are mutually trans. The salt crystallizes with one molecule of water. It was first reported in 1863.

According to Organic Syntheses, NH4[Cr(NCS)4(NH3)2] is prepared by treatment of molten NH4SCN (ca. 145–150 °C) with (NH4)2Cr2O7.

This salt was once widely used to precipitate primary and secondary amines as their ammonium salts. Included in the amines that effectively form crystalline precipitates are those derived from the amino acids, including proline and hydroxyproline. It also reacts with Hg2+ compounds, giving a red color or a red precipitate.