Quadruple bond

Source: Wikipedia, the free encyclopedia.

A quadruple bond is a type of

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.[2]

The structure of chromium(II) acetate contains a quadruple Cr–Cr bond.

History

E. Peligot, although its distinctive bonding was not recognized for more than a century.[3]

The first crystallographic study of a compound with a quadruple bond was provided by Soviet chemists for salts of Re
2Cl2−
8.

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.[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–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 octachlorodirhenate(III) anion, [Re2Cl8]2−, which features a quadruple Re–Re bond
Formation of a delta bond by the overlap of two d orbitals

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 chlorine 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–Os triple bond and a staggered 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 is ditungsten tetra(hpp).

Quadruple bonds between atoms of

dicarbon (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 recent paper by S. Shaik et al. has suggested that a quadruple bond exists in dicarbon,[8] but this is disputed.[9]

See also

References

  1. doi:10.1002/anie.200504322
    .
  2. ISBN 0-471-84997-9
    .
  3. ^
    ISBN 0-19-855649-7
    .
  4. ^ Kuznetsov, V. G.; Koz'min, P. A. "The structure of (pyH)HReCl4" Zhurnal Strukturnoi Khimii 1963, 4, 55-62.
  5. doi:10.1021/ic50025a015
    .
  6. ^
    ISBN 0-13-841891-8
    .
  7. ISBN 0935702482
    .
  8. doi:10.1038/nchem.1263
    .
  9. PMID 22354425
    .

Further reading
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