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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 common among the transition metal elements rhenium, tungsten, molybdenum, and chromium. The first chemical compound containing a quadruple bond was chromium(II) acetate, Cr2(μ-O2CMe)4(H2O)2, was synthesized in 1844 by E. Peligot, although its distinctive bonding was not recognized for much more than a century later. The quadruple bond was first characterized in potassium octachlorodirhenate(III) or K2Re2Cl8·2H2O 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 Re2Cl82- 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 K4Mo2Cl8. An example of a ditungsten compound with a quadruple bond is di-tungsten tetra(hpp). Both carbon and silicon can theoretically form quadruple bonds.
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