A sextuple bond is a type of covalent bond involving 12 bonding electrons. The only known molecules with true sextuple bonds (order 6) are diatomic Mo2 and W2, in the gaseous phase at very low temperatures. 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 (quintuple bond). There is strong evidence to believe that no two elements in the periodic table can form a bond with greater order than 6.
Mo2 can be observed in the gas phase at low temperatures (7K) by a laser evaporation technique using molybdenum sheet with for instance near infrared spectroscopy or UV spectroscopy. Like dichromium, a singlet state is expected from dimolybdenum. Higher bond order is reflected in shorter bond length of 1.94 Å.
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