Halides of Mg, Ca, Sr and Ba

    The fluorides ofMg(II), Ca(II), Sr(II) andBa(II) are ionic, have high melting points, and are sparingly soluble in water, the solubility increasing slightly with increasing cation size (Ksp for MgF₂, CaF₂ , SrF₂ and BaF₂ = 7.42 × 10 ^-11, 1.46 × 10^{- 10}, 4.33 × 10 ^-9  and 1.84 ×10 ^-7 respectively). Whereas MgF₂ adopts a rutile lattice  CaF₂, SrF₂ and BaF₂ crystallize with the fluorite structure.

  In contrast to the behaviour of BeF₂, none of the later metal fluorides behaves as a Lewis acid. The structures of gaseous group 2 metal fluoride and later halide molecules are summarized in Table 1.1 and are the subject of ongoing theoretical interest. The term ‘quasilinear’ refers to a species for which the calculated energy difference between linear and bent structures (with a change in angle of >208) is less than 4 kJ mol^-1. The most bent of the dihalides is BaF₂.

It has a bond angle in the region of 110–1268 (values come from a range of theoretical and experimental data) and the calculated energy to convert bent BaF₂ to a linear molecule is ≈21 kJ mol^-1. The preference for bent structures for the heaviest metals combined with F, Cl or Br (see Table 1.1) has been explained in terms of both ‘inverse (or core) polarization’ and the participation of d atomic orbitals for Ca, Sr and Ba. Inverse polarization occurs when the metal ion is polarizable and is polarized by F^- or Cl^-, or to a lesser extent, by Br^-. This is represented in below diagram.   The polarization is termed ‘inverse’ to distinguish it from the polarization of a large, polarizable anion by a cation.

    An alternative explanation focuses on the participation of d orbitals in the bonding in CaX₂, SrX₂ and BaX₂. Table 1.1 shows that Be and Mg form only linear gaseous dihalides.  These two metals have only s and p atomic orbitals available for bonding and the best M_X orbital overlap is achieved for a linear molecule. This is shown in below diagram for an np orbital on M with the out-of-phase combination of X…….X orbitals. For Ca, Sr and Ba, vacant 3d, 4d and 5d orbitals, respectively, are available, but can only overlap efficiently with orbitals on the X atoms if theMX₂ molecule is bent. Two interactions must be considered as shown in below diagram (the axes are defined arbitrarily as shown). The out-of-phase combination of X…..X orbitals only overlaps efficiently with the dyz orbital of M if the MX₂ molecule is bent; opening the molecule up to a linear shape ‘switches off’ this orbital interaction.

     Although the interaction between the metal dz² orbital and the in-phase combination of X…..X orbitals is most efficient when MX₂ is linear, it is still effective when the molecule is bent. The inverse polarization and participation of d atomic orbitalsmay both contribute to the problem of bent MX₂ molecules, and the explanation for the trend in shapes listed in Table 1.1 remains a matter for debate.†

Magnesium chloride, bromide and iodide crystallize from aqueous solution as hydrates which undergo partial hydrolysis when heated. The anhydrous salts are, therefore, prepared by reaction in below.

Anhydrous MCl₂, MBr₂ and MI₂ (M= Ca, Sr and Ba) can be prepared by dehydration of the hydrated salts. These anhydrous halides are hygroscopic and CaCl₂  is used as a laboratory drying agent and for road de-icing. In the solid state, many of the anhydrous halides possess complicated layer structures such as the CdI₂ lattice. Most of these halides are somewhat soluble in polar solvents such as ethers or pyridine, and a number of crystalline complexes have been isolated.

Octahedral coordination has been confirmed by X-ray diffraction studies of complexes including trans-[MgBr₂(py)₄], trans-[MgBr₂(THF)₄], cis-[MgBr₂(diglyme)(THF)] (Figure 1.1a) and trans-[CaI₂(THF)₄].

In [MgBr₂(THF)₂], octahedral coordination in the solid state is achieved by the formation of a chain structure (Figure 1.1b); py = pyridine, THF = tetrahydrofuran. The larger sizes of the heavier metals permit higher coordination numbers, e.g. pentagonal bipyramidal trans-[SrBr₂(py)₅], and trans-[SrI₂(THF)₅]. In organic chemistry, MgBr₂ is used as a catalyst for esterification reactions, and MgBr₂.2Et₂O is commercially available, being a catalyst for the conversion of aliphatic epoxides to the corresponding ketones.