Valence Angles in Group II Dihalide Molecules

Group II A atoms Ca, Sr and Ba form dihalide molecules some of which are linear and others bent. Two general models are considered to account for this. In the first we suppose a purely ionic structure in which the atoms are polarized by the resulting internal electric field. The polarization energy varies with valence angle; but it is shown that if this model were the correct one, all these molecules should be linear. In the second model we consider the valence states that arise from excitations s2 → sp and s2 → sd. The first of these would lead to a linear molecule, the second to a bent one. Considerations of the energies of excitation to these two valence states together with a similar situation for the positive ions show that bent molecules are favored when the central atom is heavy and when the attached halogens are as electronegative as possible. The experimental results are thus completely explained.