Aquadruple bond is a type ofchemical bond between twoatoms involving eightelectrons. This bond is an extension of the more familiar types ofcovalent bonds:double bonds andtriple bonds.[1] Stable quadruple bonds are most common among thetransition metals in the middle of thed-block, such asrhenium,tungsten,technetium,molybdenum andchromium. Typically theligands that support quadruple bonds areπ-donors, notπ-acceptors. Quadruple bonds are rare as compared todouble bonds andtriple bonds, but hundreds of compounds with such bonds have been prepared.[2]
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Chromium(II) acetate, Cr2(μ-O2CCH3)4(H2O)2, was the firstchemical compound containing a quadruple bond to be synthesized. It was described in 1844 byE. Peligot, although its distinctive bonding was not recognized for more than a century.[3]
The firstcrystallographic study of a compound with a quadruple bond was provided by Soviet chemists for salts ofRe
2Cl2−
8.[4] The very short Re–Re distance was noted. This short distance (and the salt'sdiamagnetism) indicated Re–Re bonding. These researchers, however, misformulated theanion as a derivative of Re(II), i.e.,Re
2Cl4−
8.
Soon thereafter,F. Albert Cotton and C. B. Harris reported the crystal structure ofpotassium octachlorodirhenate or K2[Re2Cl8]·2H2O.[5] 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.[3] The rhenium–rheniumbond length in this compound is only 224 pm. Inmolecular orbital theory, the bonding is described as σ2π4δ2 with onesigma bond, twopi bonds and onedelta bond.
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The [Re2Cl8]2−ion adopts aneclipsed 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 Re–Re axis and lie in between the Re–Cl bonds. The d orbitals directed along the Re–Cl bonds are stabilized by interaction with chloride ligand orbitals and do not contribute to Re–Re bonding.[6] In contrast, the [Os2Cl8]2− ion with two more electrons (σ2π4δ2δ*2) has an Os–Ostriple bond and astaggered geometry.[6]
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 K4[Mo2Cl8] (potassium octachlorodimolybdate).[7] An example of a ditungsten compound with a quadruple bond isditungsten tetra(hpp).
Quadruple bonds between atoms ofmain-group elements are unknown. For thediatomic carbon (C2) 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 π-bonding set of orbitals. This adds up to a bond order of 2, meaning that there exists a double bond between the twocarbon atoms. Themolecular 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,[8] but this is disputed.[9]