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. Stable quadruple bonds are most common among the transition metals in the middle of the d-block, such as 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 or K2[Re2Cl8]·2H2O by researchers in the USSR and then F. A. Cotton. 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 pi 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). An example of a ditungsten compound with a quadruple bond is di-tungsten tetra(hpp).
Valence bond theory predicts a quadruple bond as the only way to satisfy the octet rule for carbon in dicarbon. However, molecular orbital theory shows that there are two sets of paired electrons in the sigma system (one bonding, one antibonding), and two sets of paired electrons in a degenerate pi bonding set of orbitals. This adds up to give a bond order of 2, meaning that there exists a double bond between the two carbons in a C2 molecule. This is surprising because the MO diagram of diatomic carbon would show that there are two pi bonds and no sigma bonds.
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