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      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 d-block, such as rhenium, tungsten, technetium, molybdenum and chromium. Typically the ligands that support quadruple bonds are π-donors, not π-acceptors. Quadruple bonds are rare as compared to double bonds and triple bonds, but hundreds of compounds with such bonds have been prepared.


      History


      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 Re2Cl2−8. 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., Re2Cl4−8.
      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.


      Structure and bonding



      The [Re2Cl8]2− 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 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. In contrast, the [Os2Cl8]2− ion with two more electrons (σ2π4δ2δ*2) has an Os–Os 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 K4[Mo2Cl8] (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 (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 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.


      See also


      Bond order


      References




      Further reading


      Cotton, F. A.; Harris, C. B. (1965). "The Crystal and Molecular Structure of Dipotassium Octachlorodirhenate(III) Dihydrate, K2[Re2Cl8]·2H2O". Inorg. Chem. 4 (3): 330–333. doi:10.1021/ic50025a015.

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    Why doesn't CO have a quadruple bond? : r/chemhelp

    ah yes. carbon quadruple bond. : r/chemistrymemes

    ah yes. carbon quadruple bond. : r/chemistrymemes

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    Can carbon have quadruple bonds? - Chemistry Stack Exchange

    Aug 13, 2015 · s orbitals have spherical symmetry, and p orbitals have a dumb-bell shape. When these orbitals overlap, then they form the bonding orbitals (and corresponding antibonding orbitals). The p-orbitals are aligned along Cartesian (xyz) axes. If all three p-orbitals overlap, then the s-orbitals can't get 'close enough' to overlap and form a fourth bond.

    Bonding in diatomic C2, a carbon-carbon quadruple bond?

    Jun 5, 2012 · There is a recent report (2012) that carbon forms a quadruple bond in diatomic carbon, $\ce{C2}$. The excerpt below is taken from that report. The fourth bond seems pretty odd to me. $\ce{C2}$ and its isoelectronic molecules $\ce{CN+}$, BN and $\ce{CB-}$ (each having eight valence electrons) are bound by a quadruple bond.

    hydrocarbons - Why can two carbon atoms not form more than …

    Mar 20, 2017 · The latter is particularly interesting because the fourth — δ, in analogy to d — bond can be cleaved by irradiation causing a structural change that clearly shows there must have been a quadruple bond previously. To the best of my knowledge, quadruple bonds have not been shown to exist between main group elements though.

    Can carbon be the central atom of a complex? Can it form a …

    Feb 12, 2018 · Long story short, it does exist, but doesn't have a quadruple bond. (Such bonds are known, but not in $\ce{C2}$; they have a delta component , and that requires d orbitals). The problem with your description is that the sigma-oriented 2p orbital of one carbon will interact not just with the similar 2p , but also with the 2s orbital of the other.

    What is the highest known bond order? - Chemistry Stack Exchange

    Sep 17, 2015 · Only the three half-filled or empty p orbitals can mix, so the highest bond order achieved by carbon is 3. Based on this reasoning, we can imagine the element Gd, which has 8 half-filled orbitals (a 5d orbital and all seven 4f orbitals), would be the absolute limit with a bond order of 8. However, there are yet more limits on how high bond ...

    What is the binding mode of the ligand in complexes with …

    Aug 30, 2019 · The four chlorine atoms will bond using p orbitals (but the orbital type does not matter as we will only be considering σ bonds). They transform as $\mathrm{a_1+b_1+e}$ (calculation not shown). Immediately, we notice that $\mathrm d_{xy}$ does not contribute to bonding to the ligands but only to the $\ce{Re-Re}$ quadruple bond.

    Carbon with 5 bonds? - Chemistry Stack Exchange

    Jan 27, 2013 · Carbon cannot have more then 4 double-electron bonds in reasonable conditions. However, in can form a bond with 5 or 6 atoms, like $\ce{Fe6C}$ fragment, where iron atoms form octahedron around the carbon atom. However, the sum of …

    valence bond theory - OCN- has FOUR resonance structures?

    But then it said calculations show that the contributions are 61%, 30%, and 4% for the structures from left to right. Those three numbers only add up to 95%. My professor said that the remaining 5% comes from a CN Quadruple bond and a zero-order bond with oxygen. How does that work?

    Are there any molecules with delta bonds in their ground states?

    Chromium(II) acetate has a Cr-Cr quadruple bond - and hence a $\delta$ bond - in its ground state. This sort of "paddlewheel" configuration, with four carboxylic acids coordinating a pair of metal ions, is reasonably common (see also copper(II) acetate, for instance), although the extent to which the metals can be considered covalently bonded ...

    What would follow in the series sigma, pi and delta bonds?

    The concept of the bond is actually quite old and I was able to track it down to at least 1984, where Bruce E. Bursten and Geoffrey A. Ozin "demonstrate[d] for the first time the existance of φ bonds between metal atoms."