Success and failure of polarized-ion models: Bending and atomization energy of groups 2 and 12 dihalides

Polarized-ion models—with and without dipole reaction fields and charge-quadrupole interaction—are evaluated in calculating atomization energies, bond angles, and bending force constants of groups 2 and 12 dihalides. A study of the sensitivity to changes in the input parameters reveals a strong dependence on dipole polarizabilities. The charge–quadrupole bending terms of a multipole expansion are important for quasi-linear molecules, where the preceding terms in the expansion tend to cancel each other. They normally stabilize the linear geometry, but enhance bent structures, if a certain criterion for the ratio of quadrupole polarizabilities of metal and halide ions is fulfilled. The models explain the bent alkaline earth halides, but fail spectacularly for the group 12 halides—HgBr2 and HgI2 are even calculated as unstable relative to the constituent atoms. The unexpected weakness of ionic bonds is related to the large increase of the ionization energies by relativistic effects and the lanthanide contraction.