Quadruple bond

A quadruple bond is a type of chemical bond between two atoms involving 8 electrons. This bond is an extension of the more familiar types double bonds and triple bonds. 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.

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. 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 2.24 Angstrom. In the terminology of molecular orbital theory, the bonding is described as σ2π4δ2 with one sigma bond, two π bonds and one delta bond. The strength of the delta bond is a matter of debate. The bond order for the [Re2Cl8]2− is calculated to be 3.2, not 4.

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]. An example of a ditungsten compound with a quadruple bond is di-tungsten tetra(hpp).