Simple compounds

Key point: The binary compounds of the Group 2 metals contain the cations of the elements and exhibit predominantly ionic bonding. All the elements occur as M(II) in their simple compounds, which is consistent with their ns2 valence-electron configuration. Apart from Be, their compounds are predominantly ionic. With the exception of Be, the Group 2 elements form ionic (saline) hydrides; the anion present is the hydride ion, H–. In contrast, beryllium hydride adopts a three-dimensional network of linked BeH4 tetrahedra. Magnesium hydride, MgH₂, loses hydrogen when heated above 250°C and is being studied as a hydrogen-storage material. The hydrides react with water to produce hydrogen gas. All the elements form halides, MX₂, by direct combination of the elements. The halides of the elements other than Be, however, are normally formed from solution, such as by reaction of the metal hydroxide or carbonate with a hydrohalic acid (HX (aq), X = Cl, Br, I) followed by dehydration of the resulting hydrous salt. The fluorides of the larger cations (from Ca to Ba) adopt the (8,4)-coordinate fluorite structure (Fig. 12.2), but MgF₂ crystallizes with a rutile structure. The beryllium halides form covalently bonded networks of edge- or corner-linked tetrahedra. Beryllium oxide, BeO, is a white, insoluble solid with the wurtzite structure with (4,4)-coordination, as expected for the small Be2 ion; the oxides of the other Group 2 elements all adopt the rock-salt structure with (6,6)-coordination. Magnesium oxide is insoluble but reacts slowly with water to form Mg (OH)₂; likewise, CaO reacts with water to form the partially soluble Ca (OH)₂. The oxides of Sr and Ba, SrO and BaO, dissolve in water to form the strongly basic hydroxide solutions:

Magnesium hydroxide, Mg(OH)₂ , is basic but only very sparingly soluble; beryllium hydroxide, Be (OH)₂, is amphoteric and in strongly basic solutions it forms the tetrahy droxyberyllate ion, Be(OH)₄^2- :

The sulfides can be prepared by direct reaction of the elements and adopt the rock-salt structure for all except Be, which has a sphalerite structure, Fig. 3.34. Beryllium carbide, Be₂C, has an antifluorite structure formally containing Be₂ and C₄ ions. The carbides of the other members of the group have the formula MC₂ and contain the dicarbide (acetyl ide) anion, C₂^2–; they react with water to generate ethyne, C₂H₂. The elements Mg–Ra react directly with nitrogen when heated to produce the nitrides M₂N₃, which react with water to produce ammonia.With the exception of the fluorides, the salts of singly charged anions are usually soluble in water, although beryllium salts—once again on account of the highly polarizing nature of the Be2 ion—often hydrolyse in aqueous solutions with the formation of [Be(OH₂)₃ (OH)] and H₃O. The radium halides are the least soluble of the group halides: this property is used to extract radium using fractional crystallization. In general, the salts of the Group 2 elements are generally much less soluble in water than those of Group 1 on account of the higher lattice enthalpies of structures containing doubly charged cations, especially when they are in combination with highly charged anions: the carbonates, sulfates, and phosphates are insoluble or only sparingly soluble. The carbonates and sulfates of the Group 2 elements have important roles in natural water systems, rock formation, and as materials for forming hard structures. The carbonates and sulfates are insoluble, as a result of the high lattice energy of structure formed from 2 and 2 ions. The solubility of calcium carbonate increases if CO₂ is dissolved in the water, as in rainwater, due to the formation of HCO₃ with its lower charge. ‘Temporary hardness’ of water is caused by the presence of magnesium and calcium hydro gencarbonates; the cations are precipitated as carbonates on boiling solutions containing the hydrogen carbonates. Calcium carbonate is widely used by living organisms in the construction of hard structural biomaterials such as shells, bones. When heated, an alkaline earth carbonate decomposes to the oxide, although for Sr and Ba this decomposition process requires temperatures above 800°C. Calcium sulfate is used widely in the construction industry (plaster) and occurs naturally as gypsum, which is the dihydrate, CaSO₄.2H₂O. The Group 2 cations form complexes with charged polydentate ligands, such as the ana lytically important ethylenediaminetetraacetate ion (edta, see Table 7.1) and crown and crypt ligands. The most important macrocyclic complexes are the chlorophylls, which are porphyrin complexes of Mg and are involved in photosynthesis (Section 27.10d). Beryllium forms an extensive series of organometallic compounds. Alkyl- and aryl magnesium halides are very well known as Grignard reagents and are widely used in synthetic organic chemistry, where they behave as a source of alkyl and aryl anions.

Figure 12.2 The fluorite structure adopted by CaF₂, SrF₂, BaF₂, and SrCl₂.

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مواضيع ذات صلة

Partially crystalline carbon

Fullerene metal complexes

Carbon clusters

Diamond and graphite

Occurrence and extraction

Thermodynamic considerations

The atom and its ions

Oxidation states

Occurrence

Enthalpies of atomization

Electronegativity

Ionization energies and electron affinities