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 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.
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 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 2012 paper by S. Shaik et al. suggests that a quadruple bond exists in dicarbon, but this is disputed.
See also
- Bond order
References
- Radius, U.; Breher, F. (2006). "'To Boldly Pass the Metal–Metal Quadruple Bond". Angew. Chem. Int. Ed. 45 (19): 3006–3010. doi:10.1002/anie.200504322. PMID 16642513.
- Cotton, F. Albert; Wilkinson, Geoffrey (1988). "23. Metal-to-metal bonds and metal atom clusters". Advanced Inorganic Chemistry (5th ed.). John Wiley and Sons. p. 1089. ISBN 0-471-84997-9.
- Cotton, F. A.; Walton, R. A. (1993). Multiple Bonds Between Metal Atoms. Oxford: Oxford University Press. ISBN 0-19-855649-7.
- Kuznetsov, V. G.; Koz'min, P. A. "The structure of (pyH)HReCl4" Zhurnal Strukturnoi Khimii 1963, 4, 55-62.
- 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.
- Miessler, G. L.; Tarr, D. A. (1999). Inorganic Chemistry (2nd ed.). Prentice-Hall. p. 531. ISBN 0-13-841891-8.
- Girolami, G. S.; Rauchfuss, T. B.; Angelici, R. J. (1999). Synthesis and Technique in Inorganic Chemistry. Mill Valley, CA: University Science Books. ISBN 0935702482.
- Shaik, Sason; Danovich, David; Wu, Wei; Su, Peifeng; Rzepa, Henry S.; Hiberty, Philippe C. (2012). "Quadruple bonding in C2 and analogous eight-valence electron species". Nature Chemistry. 4 (3): 195–200. Bibcode:2012NatCh...4..195S. doi:10.1038/nchem.1263. PMID 22354433.
- Grunenberg, Jörg (2012). "Quantum chemistry: Quadruply bonded carbon". Nature Chemistry. 4 (3): 154–155. Bibcode:2012NatCh...4..154G. doi:10.1038/nchem.1274. PMID 22354425.
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.
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 p donors not p acceptors Quadruple bonds are rare as compared to double bonds and triple bonds but hundreds of compounds with such bonds have been prepared The structure of chromium II acetate contains a quadruple Cr Cr bond HistoryChromium II acetate Cr2 m 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 Re2 Cl2 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 Re2 Cl4 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 s2p4d2 with one sigma bond two pi bonds and one delta bond Structure and bondingThe octachlorodirhenate III anion Re2Cl8 2 which features a quadruple Re Re bondFormation 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 chloride ligand orbitals and do not contribute to Re Re bonding In contrast the Os2Cl8 2 ion with two more electrons s2p4d2d 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 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 p 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 alsoBond orderReferencesRadius U Breher F 2006 To Boldly Pass the Metal Metal Quadruple Bond Angew Chem Int Ed 45 19 3006 3010 doi 10 1002 anie 200504322 PMID 16642513 Cotton F Albert Wilkinson Geoffrey 1988 23 Metal to metal bonds and metal atom clusters Advanced Inorganic Chemistry 5th ed John Wiley and Sons p 1089 ISBN 0 471 84997 9 Cotton F A Walton R A 1993 Multiple Bonds Between Metal Atoms Oxford Oxford University Press ISBN 0 19 855649 7 Kuznetsov V G Koz min P A The structure of pyH HReCl4 Zhurnal Strukturnoi Khimii 1963 4 55 62 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 Miessler G L Tarr D A 1999 Inorganic Chemistry 2nd ed Prentice Hall p 531 ISBN 0 13 841891 8 Girolami G S Rauchfuss T B Angelici R J 1999 Synthesis and Technique in Inorganic Chemistry Mill Valley CA University Science Books ISBN 0935702482 Shaik Sason Danovich David Wu Wei Su Peifeng Rzepa Henry S Hiberty Philippe C 2012 Quadruple bonding in C2 and analogous eight valence electron species Nature Chemistry 4 3 195 200 Bibcode 2012NatCh 4 195S doi 10 1038 nchem 1263 PMID 22354433 Grunenberg Jorg 2012 Quantum chemistry Quadruply bonded carbon Nature Chemistry 4 3 154 155 Bibcode 2012NatCh 4 154G doi 10 1038 nchem 1274 PMID 22354425 Further readingCotton 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
