chromium(II) acetate
Other names: chromous acetate, chromium diacetate
Cr2(OAc)4
46/50 
rare
Chromium(II) acetate is a striking compound notable for its beautiful red-pink color and remarkable bonding pattern. This dimeric complex features one of the most studied examples of a metal-metal quadruple bond, with two chromium atoms bound directly to each other and bridged by four acetate ligands in a paddlewheel structure. The quadruple bond consists of one σ bond, two π bonds, and a δ bond formed by the overlap of dxy orbitals on each chromium center. The salt is a red-brown solid, with a distinctive tabular crystal structure as a dihydrate salt. Highly air-sensitive, chromium(II) acetate rapidly oxidizes to chromium(III) species when exposed to oxygen, changing from pink-red to green-brown. The salt has found applications in organic synthesis as a selective reducing agent and has been extensively studied as a model system for metal-metal multiple bonding in coordination chemistry.