CAN ADDITION OF A BONDING ELECTRON WEAKEN A BOND

The behavior of Li2 upon ionization can be rationalized as follows. As in the simple MO bonding model, removal of a bonding electron reduces the electron density between the nuclei, causing a bond length increase. The presence of polarization, particularly in the ion, partly compensates. The interplay of effects is such that the extra electron repulsion in Li2, particularly the core/valence interactions enhanced by the high penetration of the valence electrons, causes Li2 to be less stable than expected. The greater importance of polarization effects in Li2+ compared with Li2 helps to stabilize the Li2+ bond and to enhance the attraction between the nuclei at long range, thus producing a lower force constant. All the alkali dimers will be susceptible to the effects discussed above, and so the anomalous behavior in all of them is not surprising. This does not extend to the H2/H2+ system, however, due to two main differences. Hydrogen atoms have much lower polarizabilities than the alkali metals, giving reduced polarization stabilization in H2+. Also, the absence of core electrons excludes the core/valence electron repulsion. Thus the two main advantages in stability possessed by Li2+ compared with Li2 are not available to H2+. Other diatomics, e.g., N2, have lower polarization effects and their bonds are usually shorter and stronger. It is only in weakly bonded systems like the alkali-metal dimers that the small effects of polarization and core/valence electron repulsion become significant. In the majority of chemical systems these effects do not produce qualitative anomalies like the Li2+-Li2 binding energy difference. Nevertheless, it is clear that the approximate nature of the simple bonding model must be remembered and due care taken in its application.