Sextuple bond

MO diagram of dimolybdenum

A sextuple bond is a type of covalent bond involving 12 bonding electrons and in which the bond order is 6. The only known molecules with true sextuple bonds are the diatomic dimolybdenum (Mo₂) and ditungsten (W₂), which exist in the gaseous phase and have boiling points of 4,639 °C (8,382 °F) and 5,930 °C (10,710 °F). There is strong evidence to believe that there is no element with atomic number below about 100 that can form a bond with a greater order than 6 between its atoms,^[1] but the question of possibility of such a bond between two atoms of different elements remains open.

Dimolybdenum and ditungsten

Dimolybdenum (Mo₂) can be observed in the gas phase at low temperatures (7 K) by a laser evaporation technique using molybdenum sheet with, for instance, near infrared spectroscopy or UV spectroscopy.^[2] Like dichromium, a singlet state is expected from dimolybdenum.^[3] Higher bond order is reflected in shorter bond length of 194 pm.

Other molecules

Although diatomic Cr₂ and U₂ have formal structures with twelve-electron bonds, their effective bond orders derived from quantum chemistry calculations are less than 5 (which would be quintuple bonds).^[1]

References

1. Roos, Björn O.; Antonio C. Borin; Laura Gagliardi (2007). "Reaching the Maximum Multiplicity of the Covalent Chemical Bond" (PDF). Angewandte Chemie International Edition 46 (9): 1469–72. doi:10.1002/anie.200603600. PMID 17225237.
2. D. Kraus, M. Lorenz and V. E. Bondybey (2001). "On the dimers of the VIB group: a new NIR electronic state of Mo₂". PhysChemComm 4 (10): 44–48. doi:10.1039/b104063b.
3. Gabriel Merino, Kelling J. Donald, Jason S. D’Acchioli, and Roald Hoffmann (2007). "The Many Ways To Have a Quintuple Bond". J. Am. Chem. Soc. 129 (49): 15295–15302. doi:10.1021/ja075454b. PMID 18004851.

Further reading

• Bruce E. Bursten; F. Albert Cotton; Michael B. Hall (1980). "Dimolybdenum: nature of the sextuple bond". Journal of the American Chemical Society 102 (20): 6348. doi:10.1021/ja00540a034.
• Borin, A; Gobbo, João Paulo; Roos, Björn O. (2008). "A theoretical study of the binding and electronic spectrum of the Mo2 molecule". Chemical Physics 343 (2–3): 210. Bibcode:2008CP....343..210B. doi:10.1016/j.chemphys.2007.05.028.
• Marvin M. Goodgame; William A. Goddard I I I (1981). "The "sextuple" bond of chromium dimer". The Journal of Physical Chemistry 85 (3): 215. doi:10.1021/j150603a001.
• Chisholm, Mh (Feb 2007). "Metal to metal multiple bonds in ordered assemblies" (Free full text). Proceedings of the National Academy of Sciences of the United States of America 104 (8): 2563–70. Bibcode:2007PNAS..104.2563C. doi:10.1073/pnas.0610364104. PMC 1815223. PMID 17299047.
• Norman, Joe G., Jr.; Ryan, P. Barry (1980). "Metal-metal bond energies in diatomic molybdenum, octachloromolybdate (Mo2Cl84-), and molybdenum formate (Mo2(O2CH)4)". Journal of Computational Chemistry 1 (1): 59–63. doi:10.1002/jcc.540010107.
• Atha, P. M.; Hillier, I. H.; Guest, M. F. (1980). "Electron correlation and the nature of the sextuple bond in the dimolybdenum molecule". Chemical Physics Letters 75 (1): 84–86. Bibcode:1980CPL....75...84A. doi:10.1016/0009-2614(80)80469-6.
• Wood, Carol; Doran, Mark; Hillier, Ian H.; Guest, Martyn F. (1980). "Theoretical study of the electronic structure of the transition metal dimers, discandium, dichromium, dimolybdenum, and dinickel". Faraday Symp. R. Soc. Chem. 14 (Diatomic Met. Met. Clusters): 159–169. doi:10.1039/fs9801400159.