Ab initio molecular orbital theory has been used in a systematic study of the firstand second-row dimer dications He?+, H3NNH32+, H200H22+, HFFH, Ne22+, H3PPH32+, H2SSH22+, HC1C1H2+, and Ar22+. Although all nine systems are thermodynamically unstable with respect to symmetric fragmentation into two monocations, potential energy barriers inhibiting such fragmentations are predicted to exist for all of the dimers except HFFH2+, Ne?+, and Ar22+. In particular, the hydrazinium and diphosphinium dications are found to be very stable kinetically with respect to both symmetric fragmentation and deprotonation. The equilibrium A-A bond lengths in the dimer dications (A?') are all significantly shorter than in the corresponding (hemibonded) monocations (A2'+), but the fragmentation barriers are usually smaller. The equilibrium structures of the dimer dications are found to be very similar to those of their isoelectronic, isostructural, neutral counterparts. The usefulness of the recently introduced A parameter in understanding the stabilities and fragmentation processes of all nine dications is emphasized. The chemistry of gas-phase dications is progressing rapidly on both theoretical and experimental fronts.' Because the total nuclear (positive) charge in such ions exceeds the total electronic (negative) charge by two electronic units, there is a very strong tendency, in almost all of these systems, for a coulomb explosion to occur, leading to the formation of two monocations. Despite this, many dications are metastable species whose lifetimes are sufficiently great that they may be studied experimentally. Indeed, some are even thermodynamically stable; that is, they are lower in energy than any of their possible fragmentation products and are therefore indefinitely stable in isolation. In a previous study,2 we have investigated the family of three-electron hemibonded A2*+ dimers where A is one of the normal-valent hydrides He, NH3, H 2 0 , HF, Ne, PH3, HIS, HCI, or Ar and in which the monomeric units are bonded through the heavy atoms. We have found that the hemibond strengths in such species are remarkably strong (>IO0 kJ mol-]). The formation of stable monocations of this type is not unexpected because, in ionizing a neutral A, dimer, an electron is removed from an antibonding orbital, leading to the formation of a three-electron bond (Figure 1). For example, ionization in the He2 system changes a repulsive four-electron interaction in neutral He2 to a bonding three-electron interaction (with a formal bond order of in He;+. It is natural then to inquire whether the removal of a second antibonding electron, leading to the formation of a dicationic A?+ dimer, will lead to an even stronger A-A bond (with, a formal bond order of unity, Figure 1). Although it is likely that, in some of the dications, the enhanced covalent binding may be partially offset by the substantial coulombic repulsion, which will be present in such species, the counterintuitive theoretical finding that a bond may be strengthened by effective charges on the adjacent n ~ c l e i , ~ and the fact that both the He22+ dication4 and the N2H62+ dication5 have already been observed experimentally, encouraged us to undertake a detailed study of the complete family of A22+ dimers (where A is one of the normal-valent firstor second-row hydrides, as above). Some of the main properties of the A2*+ dimer dications, e.g. their equilibrium structures, would be expected to be well described by conventional theoretical procedures. On the other hand, it is ~ e l l k n o w n ~ * ~ that the theoretical study of the fragmentation of (1 ) For a recent review, see: Koch, W.; Schwarz, H. Structure/Reactiuity and Thermochemistry of Ions; Lias, S. G., Ausloos, P., Eds.; Reidel: Dordrecht, The Netherlands, 1987. (2) Gill, P. M. W.; Radom, L. J . Am. Chem. SOC. 1988, 110, 4931. (3) Dunitz, J . D.; Ha, T. K. J . Chem. Soc., Chem. Commun. 1972, 568. (4) Guilhaus, M.; Brenton, A. G.; Beynon, J. H.; Rabrenovic, M.; Schleyer, P. v. R. J . Phys. B 1984, 17, L605. ( 5 ) See, for example: Frlec, B.; Gantar, D.; Golic, L.; Leban, I. Acta Crystallogr. 1981, 37, 666. (6) Schleyer, P. v. R. Adu. Mass. Spectrom. 1985, 287. (7) See, for example: Taylor, P. Mol. Phys. 1983, 49, 1297. Table I. Basis Set Dependence of the Calculated Equilibrium Bond Length (rq, A), Transition Structure Bond Length (rn, A), Dissociation Barrier (De*, k J mol-I), and A Value (kJ mol-I) for the Dication basis set r, rm De' Ab STO-3G 3-2 1 G 6-31G 6-311G (1 0 s ) d 6-31 lG(d,p)' 6-31 lG(MC)(d,p)' 6-31 lG(MC)(d,2p)' 6-31 lG(MC)(d,3p)' 6-31 lG(MC)(d,3pd)'a 6-31 lG(MC)(d,3~2dYg 6-31 lG(MC)(d,3~2dlf)'-