Rhodium(II) acetate

Rhodium(II) acetate is the coordination compound with the formula Rh2(AcO)4, where AcO− is the acetate ion. This dark green powder is slightly soluble in polar solvents, including water. It is used as a catalyst for cyclopropanation of alkenes. It is a widely studied example of a transition metal carboxylate complex.

Preparation
Rhodium(II) acetate is usually prepared by the heating of hydrated rhodium(III) chloride in a methanol-acetic acid mixture. The crude product is the bis(methanol) complex, but it is easily desolvated.

Structure and properties
The structure of rhodium(II) acetate features a pair of rhodium atoms, each with octahedral molecular geometry, defined by four acetate oxygen atoms, water, and a Rh–Rh bond of length 2.39 Å. The water adduct is exchangeable, and a variety of other Lewis bases bind to the axial positions. Copper(II) acetate and chromium(II) acetate adopt similar structures.

Chemical properties
The dimer binds a number of classical Lewis bases to form 2:1 adducts:

Its Lewis acidity is eclipsed by the enhanced reactivity of rhodium(II) trifluoroacetate, which even binds arenes and alkenes.

The acetate group can be replaced by other carboxylates of strong acids. The yields are nearly quantitative.

Like almost all rhodium complexes, rhodium(II) acetate catalyzes many reactions such as hydrogenation and hydrosilylation. No evidence exists for mechanisms and the behavior has not motivated further work. The important observation that rhodium(II) acetate catalyzes reactions of diazo compounds has led to considerable research, but mostly focused on the trifluoroacetate or chiral derivatives.