This article will begin with an introductory look at various families of weakly bound diatomics and the more important representatives of each. The next section will discuss a uniform method for describing and comparing the interatomic potential energy functions of these species. Finally, the implications of these functions and comparisons among them will be discussed. This is not intended to be an all-encompassing review of weakly bound diatomics; -2-consequently, an occasional reader may find a potentially important molecule left unmentioned. I apologize for this limitation in advance, and reiterate that the goal of the article is to illuminate families of interactions and the variations in bonding exhibited among them.
FAMILIES OF WEAKLY BOUND DIATOMICSA cursory glance through the Periodic Table leads one to predict most of the weakly bound homonuclear diatomics. Certainly, the rare gas dimers; He2 through Rn2, are the best known examples. In fact, the interatomic potential function for Ar 2 is as well characterized 2 as that for many common stable diatomics. The next-best known family3 is the Group II metal dimers: the alkaline earth diatomics, and Zn2, Cd2, and Hg2. The ground state atoms have closed (s2) configurations, and a formal analogy to rare gas dimers is predicted. However, as one proceeds atom by atom from the dimer to the bulk, these species revert to strongly bound metals due to the presence of low-lying sp or ds configurations. The possible role such configurations may have in the dimer bond will be discussed below.Less obvious weakly bound homonuclear diatomics can be found scattered about the Periodic Table. Atomic Mn has a 3d54s2 configuration in its ground state. The stability of the d5 configuration in this first row transition element leads to an anomalously weak binding energy 4 ' 5 for Mn 2 : De ~ 0.4 ± 0.25 eV (or6 ~ 3,500 ± 2000 cm-1). Similarly, the stability of the 4f76s2 configuration of Eu and the 4fl46s2 configuration of Yb yields anomalously weak binding, 7 and the actinide dimers Pu 2 , Am 2 and Bk 2 through No 2 are expected to be weakly bound due to the bonding ineffectiveness of the highly localized Sf electrons.8 as one moves down a Periodic Group, in contrast to the behavior of chemically bound diatomics, for which the energies generally decrease.A sufficiently large number of heteronuclear dimers have been studied to allow one to note certain trends. The majority of these dimers contain at least one rare gas, and the heteronuclear rare gas dimers are the prototypes.They provided the first arena in which simple combination rules could be tested (and found to be inaccurate).9 Three other classes that are well studied are the rare gas-monohalides, the rare gas-alkali metal dimers, and certain molecular ions. If the reader would stop for a moment and picture in his or her mind the potential energy function for a favorite strongly bound diatomic graphed on the same scale as the potential function for a weakly bound diatomic, the imagined graph would probably look like the one ...