ChromiumII

Chromium(II) refers to chromium in the +2 oxidation state, commonly written as Cr2+. In this oxidation state chromium has a d4 electronic configuration and forms a variety of coordination compounds, often with four to six ligands. Cr(II) compounds are typically paramagnetic and colored, and they are especially sensitive to air and moisture, tending to be oxidized to chromium(III) under many conditions.

Properties and behavior

Cr(II) salts are generally strong one-electron reducing agents. They are more stable in non-aqueous or strictly controlled environments than in water, where aquated Cr2+ ions rapidly oxidize to Cr(III). In solution, Cr(II) complexes with water or other ligands can exhibit colors ranging from blue to green, depending on the ligands and coordination geometry. The chemistry of Cr(II) is distinguished by reversible oxidation to Cr(III) and by its tendency to form adducts with ligands such as ethers and phosphines in dry solvents.

Occurrence and minerals

In nature, chromium is mainly found in the Cr(III) oxidation state within minerals like chromite. Cr(II) species do not occur as stable free ions in geological materials and are generally generated only under laboratory conditions or within specialized synthetic environments.

Preparation and applications

Cr(II) compounds are typically prepared by reducing Cr(III) precursors (for example CrCl3) under inert, anhydrous conditions, often using zinc or other reductants in dry solvents such as THF to give Cr(II) salts like CrCl2. These reagents are widely used in organic synthesis as single-electron reductants, capable of generating radical intermediates and enabling various transformations under mild conditions. Cr(II) chemistry requires careful handling due to air sensitivity and potential toxicity.

Safety and handling

Cr(II) compounds are toxic and strong reducing agents. They should be handled under inert atmosphere in appropriate glassware and disposed of as hazardous waste in accordance with regulations.