2007

The maximum multiplicity of the covalent chemical bond

The research in the group of Professor Laura Gagliardi, Département de Chimie Physique, is based on the use of computational quantum chemistry to study properties of molecules in their electronic ground state and excited states.
The concept of bond order and in particular the possibility of multiple bonding between atoms in a molecule are concepts that have always fascinated chemists.1 Theoretical and experimental work has challenged old chemical paradigms concerning the possible multiplicity that can be achieved in a chemical bond. At the same time, the concept of a multiple bond is not clearly defined and there is a need for a more quantitative measure. The group of Laura Gagliardi has been able to introduce such a measure based on the analysis of the wave function of the systems that have been examined, using state of the art quantum chemical methods, which treat electron correlation effects and relativistic effects.

The multiplicity of a chemical bond is determined by the number of electron pairs that occupy the region between the two bonded atoms in bonding molecular orbitals. The hydrogen molecule has, for example, a single bond with two electrons in one orbital formed from the 1s orbitals on each atom. The nitrogen molecule, N2, has a triple bond; the three unpaired 2p electrons on each atom combine to form this very strong bond. Before the work of Cotton in 1965,2 the triple bond was assumed to be the highest multiplicity that a chemical bond can have. In the past few years the Gagliardi’s group has performed a systematic study of the covalent chemical bond covering the entire periodic system. They have identified the existence of a quintuple bond3 in the U2 diatom and recently they have shown that the maximum bond multiplicity is six. The maximum value is reached in the tungsten diatom, W2.4

W2 Multiplicity
The molecular orbitals forming the tungsten-tungsten hextuple bond in W2

References :
1) Frenking, G.; Tonner, R. Nature 2007, 446, 276.
2) Cotton, F. A.; Harris, C. B. Inorg. Chem. 1965, 4, 330.
3) L. Gagliardi and B.O. Roos, Quantum chemical calculations show that the uranium molecule U2 has a quintuple bond, Nature, 2005 , 433, 848.
4) Roos, B. O.; Borin, A. C.; Gagliardi, L. Angew. Chem. Int. Ed. 2007, 46, 1469.

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8 octobre 2007
  2007