Group 12 Dihalides : Structural ACHTUNGTRENNUNG Predilections from Gases to Solids

What happens to molecules at the borderline of ionic and covalent bonding as they pass from the vapor—where they are generally monomeric and dimeric—to the solid state? This question, which we have broached for the alkaline earth dihalides, is here pursued for the Group 12 dihalides, another class of molecules at an interface of bonding types. The immense chemistry of the Group 12 metals Zn and Cd, less so Hg, is dominated by oxidation state 2. Therefore, the bonding in Group 12 is generally regarded as similar to that of the main group alkaline earth elements. However, there are good theoretical reasons to believe that the chemistry of the elements of the two groups differ in many ways. First, there is an increase in the effective nuclear charge Z on the valence s electrons as the subvalent (d or f) atomic orbitals are filled going across the periodic table from Group 2 to Group 12. Second, there is the importance of relativistic effects for the heavier transition metals. And third, there is also an essential difference in the role of the nearby d orbitals; the (n 1)d orbitals, empty in the Group 2 metals, are completely occupied in Group 12. As the d orbitals in period 4 are filled going from Sc to Zn, the effective charge seen by the valence (4s and 3d) electrons increases, and thus there is an overall reduction in the atomic radii (the so-called “d-block contraction”). This contraction phenomenon is apparent in period 5 (Y to Cd), as well, but is less pronounced in that row. In period 6, the filling of the first set of f atomic orbitals (the lanthanide 4f orbitals) has a similar effect on the radii of the transition metals, Au, and Hg. The increase in Z as the 4f orbitals are filled results in a significant contraction in the radius of these elements (the lanthanide contraction). 8] Relativistic effects, increasing roughly as Z (see section 2; ref. [4]), become increasingly important as we go down the short group Zn to Hg. The influence of relativistic Abstract: Connections between the structures of Group 12 dihalides in their vapor and crystal phases are sought and discussed. The molecular structures of all monomers and dimers (MX2: M=Zn, Cd, Hg and X =F, Cl, Br, I) were calculated at the density functional B3PW91 and MP2 computational levels. All the monomers are linear, with the mercury dihalide molecules having shorter bonds than their cadmium analogues; the ZnX2 and CdX2 structures are similar. The shorter Hg X distances are traced back to relativistic effects. For the dimers, many possible geometrical arrangements were considered. The zinc and cadmium dihalide dimers have the usual D2h-symmetry geometry, whereas the mercury dihalide dimers are loosely-bound units with C2h symmetry. The origins of this C2h structure are discussed from different points of view, including frontier orbital interactions. The crystals of Group 12 dihalides span a wide range of structure types, from three-dimensional extended solids to molecular crystals. There is an obvious connection between the structures and characteristics of monomers, their dimers, and the crystals they form. The similarities as well as startling differences from the Group 2 dihalides are analyzed.