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 of covalent bonds: double bonds and triple bonds. Stable quadruple bonds are most common among the transition metals in the middle of the , such as rhenium, tungsten, technetium, molybdenum and chromium. Typically the ligands that support quadruple bonds are ÃÂ-donors, not ÃÂ-acceptors. Quadruple bonds are rare as compared to double bonds and triple bonds, but hundreds of compounds with such bonds have been prepared.
Chromium(II) acetate, Cr<sub>2</sub>(ü-O<sub>2</sub>CCH<sub>3</sub>)<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>, 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 . The very short ReâÂÂRe distance was noted. This short distance (and the salt's diamagnetism) indicated ReâÂÂRe bonding. These researchers, however, misformulated the anion as a derivative of Re(II), i.e., .
Soon thereafter, F. Albert Cotton and Charles B. Harris reported the crystal structure of potassium octachlorodirhenate or K<sub>2</sub>[Re<sub>2</sub>Cl<sub>8</sub>]÷2H<sub>2</sub>O. 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 rheniumâÂÂrhenium bond length in this compound is only 224 pm. In molecular orbital theory, the bonding is described as ÃÂ<sup>2</sup>ÃÂ<sup>4</sup>ô<sup>2</sup> with one sigma bond, two pi bonds and one delta bond.
The [Re<sub>2</sub>Cl<sub>8</sub>]<sup>2âÂÂ</sup> ion adopts an eclipsed conformation as shown at left. The delta bonding orbital is then formed by overlap of the d orbitals on each rhenium atom, which are perpendicular to the ReâÂÂRe axis and lie in between the ReâÂÂCl bonds. The d orbitals directed along the ReâÂÂCl bonds are stabilized by interaction with chloride ligand orbitals and do not contribute to ReâÂÂRe bonding. In contrast, the [Os<sub>2</sub>Cl<sub>8</sub>]<sup>2âÂÂ</sup> ion with two more electrons (ÃÂ<sup>2</sup>ÃÂ<sup>4</sup>ô<sup>2</sup>ô*<sup>2</sup>) has an OsâÂÂOs triple bond and a staggered geometry.
Many other compounds with quadruple bonds between transition metal atoms have been described, often by Cotton and his coworkers. Isoelectronic with the dirhenium compound is the salt K<sub>4</sub>[Mo<sub>2</sub>Cl<sub>8</sub>] (potassium octachlorodimolybdate). An example of a ditungsten compound with a quadruple bond is ditungsten tetra(hpp).
Quadruple bonds between atoms of main-group elements are unknown. For the diatomic carbon (C<sub>2</sub>) molecule as an example, 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 ÃÂ-bonding set of orbitals. This adds up to a bond order of 2, meaning that there exists a double bond between the two carbon atoms. The molecular orbital diagram of diatomic carbon would show that there are two pi bonds and no sigma bonds. A 2012 paper by S. Shaik et al. suggests that a quadruple bond exists in dicarbon, but this is disputed.