Reaching the Maximum Multiplicity of the Covalent Chemical Bond

摘要

The bond order and in particular the possibility of multiple bonding between atoms in a molecule have been highlighted in two recent articles.[1], [2] Theoretical and experimental work have challenged old chemical paradigms concerning the possible multiplicity that can be achieved in a chemical bond. On the other hand, the concept of a multiple bond is not clearly defined and there is a need for a more quantitative measure. In this contribution we attempt to introduce such a measure and apply it to a number of multiply bonded systems. As a result of the analysis, we show that the highest multiplicity that can be achieved in a bond between two equal atoms is six. 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 1964, the triple bond was assumed to be the highest multiplicity that a chemical bond can have. We show here, through a systematic study of the covalent chemical bond covering the entire periodic system, that the maximum bond multiplicity is six. The maximum value is reached by the tungsten diatom, W2. No other pair of atoms in the periodic system (atomic numbers smaller than about 100) reaches a higher bond order.