Two chemists from Italy and Sweden have discovered that the uranium (U₂) molecule is joined together by five chemical bonds, the greatest number ever found between two identical atoms.

Chemists have been exploring the covalent bonds that link identical atoms for years, and have discovered that different elements can form single, double, triple or even quadruple bonds, which were identified between transition metal atoms in the 1960s. So far, however, chemists have been unable to characterise the bonds found in actinide atoms - from actinium (89) to lawrencium (103) - because of the complexity of their many valence orbitals. This is what Laura Gagliardi from the University of Palermo, Italy, and Björn Roos from the Chemical Center, Lund, Sweden, have now accomplished.

Gagliardi and Roos decided to explore bonding in actinide atoms by carrying out a computational investigation of the molecule formed by two uranium (92) atoms. Because of the complexity involved, they analysed the bonding using a quantum chemical method known as CASSCF/CASPT2, which offers maximum flexibility for describing electronic structures and is also capable of handling arbitrary spin.

The two chemists discovered that U₂ consists of three 'normal' electron-pair bonds and four weaker one-electron bonds. In total, therefore, U₂ possesses 10 bonding electrons, which means that it effectively has a quintuple bond.

According to Gagliardi, this is the first time that a quintuple bond has been identified and it completes chemists' understanding of the covalent chemical bonds between identical atoms. 'It may also have practical consequences in the sense that future experiments will be undertaken on U₂ or other diactinide molecules, for example Th₂ [thorium] or Pu₂ [plutonium] ,' she told Chemistry World. She adds that novel molecules with a central actinide-actinide bond might also be synthesised.

Gagliardi now plans to carry out research into other diactinide molecules, as well as compounds that contain U₂ as a central unit.

Jon Evans