Ionic character and bonding

The chemistry of the alkaline-earth metals, like that of the alkali metals, is for the most part reasonably interpreted in terms of an ionic model for the compounds formed. This model is less satisfactory for the chemistry of beryllium and magnesium than for the heavier alkaline-earth metals. In fact, most beryllium compounds are molecular (covalent) rather than ionic. This is a consequence of the high charge-to-size ratio of the Be²⁺ ion, which strongly polarizes bonds to it.

Evidence for lower-oxidation-state alkaline-earth metal compounds was controversial for many years. Some reports dating from the 1950s of “MX” halides (e.g., CaCl, SrBr) that were stable at room temperature were later demonstrated to be in error, as the compounds were actually metal hydride halides, MHX. Genuine examples of alkaline-earth compounds with metals in the +1 oxidation state were later established but were confined to species that either formed in the gas phase, such as the methyl strontium radical (∙SrCH3), or were stabilized in extremely low-temperature conditions, such as the magnesium subhalide MgCl, which can be trapped in a frozen gas matrix at 12 K (−261 °C, or −438 °F). A small number of coordination compounds are now known with the formula LMg―MgL, in which L is a chelating, anionic ligand. The compounds contain covalently bonded magnesium in a [Mg₂]²⁺ unit, ionically coordinated to the ligands. Although the metal centres are formally in the +1 oxidation state, they may also be described as divalent, given that both valence electrons of the metal centres are involved in bonding (one to the other metal, to form the M―M bond, and the other to the ligand L).

Evidence has been growing since the 1960s that the heavier members of the Group 2 elements (calcium, strontium, and barium) display structural features that are not consistent with purely ionic bonding. Molecular beam measurements have shown that the gaseous Group 2 dihalides—MF₂ (M = Ca, Sr, Ba), MCl₂ (M = Sr, Ba), and BaI₂—are nonlinear, or “bent.” There are also organometallic compounds with cyclopentadienyl rings (Cp) or alkyl ligands (R) that do not have the expected linear Cp―M―Cp or R―M―R structures.

One suggested explanation for the bending in these compounds is that the filled “semi-core” (n − 1)p and the valence (n − 1)d orbitals of the heavier alkaline-earth metals are involved in bonding (for calcium, this means the 3p and 3d orbitals; for strontium, the 4p and 4d; for barium, the 5p and 5d). The wave functions of these orbitals can mix in the process of bonding, leading to nonlinear geometries. This interpretation is supported by calculations, which must explicitly include participation by the metal d orbitals to reproduce the bent structures.

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Zinc, cadmium, and mercury, the Group 12 (IIb) elements, are often compared with the alkaline-earth elements calcium, strontium, and barium. Cadmium, for example, has the electronic configuration [Kr]4d¹⁰5s², with the 10 4d electrons taking virtually no part in chemical bonding. The 5s² electrons, however, are much less readily ionized in cadmium than they are in strontium, for the 4d electrons act as an ineffective shield for the corresponding increased charge on the cadmium nucleus. The chemistry of the Group 12 metals, therefore, is markedly less ionic than the chemistry of the alkaline-earth metals. The ionic radius of the cadmium ion Cd²⁺ (0.95 Å) is very similar to that of calcium Ca²⁺ (1.00 Å). A quantitative comparison of cadmium and calcium chemistry, therefore, clearly shows up the less ionic character of cadmium chemistry without complications due to differences in ionic size.

Somewhat closer chemical parallels exist between the heavier alkaline earths and the rare-earth elements that have readily accessible +2 oxidation states—namely, ytterbium, samarium, and europium. The ionic radii of Ca²⁺ (1.00 Å) and Yb²⁺ (1.02 Å) are similar, as are those of Sr²⁺ (1.18 Å) and Sm²⁺ (1.19 Å) or Eu²⁺ (1.17 Å). The chemical inaccessibility of the 4f electrons of the rare-earth elements means that their bonding is ionic in much the same way as that of the Group 2 metals. This is reflected in the sum of the first two ionization energies for the metals: calcium (17.98 eV) versus ytterbium (18.43 eV), and strontium (16.73 eV) versus samarium (16.7 eV) and europium (16.91 eV). This means that Eu²⁺ ions, for example, can sometimes be used as a “probe” for the alkaline-earth metals, substituting for strontium ions in situations where advantage can be taken of the spectroscopic and magnetic properties that make Eu²⁺ readily identifiable. The parallels between the alkaline-earth and rare-earth metals extend to reactivity as well; the latter react with water to release hydrogen, for example, and dissolve in liquid ammonia to produce blue solutions analogous to those from the alkaline-earth metals.

Because of radium’s intense radioactivity and high cost, its chemistry is less well-investigated than that of the other alkaline-earth metals. Many simple compounds—e.g., radium oxide (RaO)—either are poorly characterized or seem never to have been prepared. Radium salts will discolour over time, because of radiation damage of their crystal lattices. In general, the chemistry of radium is inferred by extrapolation from the chemistry of calcium, strontium, and, especially, barium.

In accordance with their electropositive character, the Group 2 metals are considered to form type a, or “hard,” ions and thus preferentially bind to ligands with hard donor atoms such as oxygen, nitrogen, or fluorine. The Group 2 ions are readily hydrated, and the number of water molecules directly attached to a metal ion is greater with the larger ions for purely steric (geometrical) reasons (typically 4, 6, and 6–8 for Be²⁺, Mg²⁺, and Ca²⁺, respectively). The strength of bonding to the water molecules decreases with increasing ionic radius, however. Large anions, such as sulfate, tend to form weak ion-pair complexes more readily with the larger metal ions of the family, but weak-acid anions, such as acetate, tend to form stronger complexes with the smaller metal ions, particularly those of magnesium and beryllium. That many of these complexes are molecular rather than ionic is shown by their ready extraction from aqueous solution (which preferentially dissolves ionic substances) into organic solvents (which dissolve molecular ones).