A quadruple bond is a type of chemical bond between two atoms involving eight electrons. This bond is an extension of the more familiar types double bonds and triple bonds.^[1] Stable quadruple bonds are most common among the middle members transition metal elements such rhenium, tungsten, molybdenum and chromium. Typically the ligands that support quadruple bonds are π-donors, not π-acceptors.

The octachlorodirhenate(III) anion, [Re₂Cl₈]²⁻, which features a quadruple Re-Re bond

Chromium(II) acetate, Cr₂(μ-O₂CMe)₄(H₂O)₂, was the first chemical compound containing a quadruple bond to be synthesized. It was described in 1844 by E. Peligot, although its distinctive bonding was not recognized for more than a century.^[2] The quadruple bond was first characterized in potassium octachlorodirhenate(III) or K₂[Re₂Cl₈]·2H₂O by F. A. Cotton in 1964. The rhenium-rhenium bond length in this compound is only 224 pm. In the terminology of molecular orbital theory, the bonding is described as σ²π⁴δ² with one sigma bond, two π bonds and one delta bond.

Many other compounds with quadruple bonds have been described, often by Cotton and his coworkers. Isoelectronic with the dirhenium compound is the salt K₄[Mo₂Cl₈] (potassium octachlorodimolybdate).^[3] An example of a ditungsten compound with a quadruple bond is di-tungsten tetra(hpp).

The structure of the hydrated diacetates of molybdenum(II) and chromium(II)

[edit] References

[edit] See also

• Covalent bond:Bond order
• Quintuple bond