Quadruple bond
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-O2CCH3)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 first crystallographic study of a compound with a quadruple bond was provided by Soviet chemists for salts of Re
2Cl2−
8. The very short Re–Re distance was noted. This short distance (and the salt's diamagnetism) indicated Re–Re bonding. These researchers however misformulated the anion as a derivative of Re(II), i.e., Re
2Cl4−
8.
Soon thereafter, F. Albert Cotton and C.B. Harris reported the crystal structure of potassium octachlorodirhenate or K2[Re2Cl8]·2H2O. This structural analysis indicated that the previous characterization was mistaken. Cotton and Harris formulated a molecular orbital rationale for the bonding that explicitly indicated a quadruple bond. The rhenium–rhenium bond length in this compound is only 224 pm. In molecular orbital theory, the bonding is described as σ2π4δ2 with one sigma bond, two pi bonds and one delta bond.
...
Wikipedia