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12.1: Introduction

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    34093
    • Beryllium is found in the mineral beryl, Be3Al2(SiO3)6. Beryl with around 2.9% Cr3+ substituting for Al3+ is emerald.
    • Beryllium compounds are dangerously toxic and can pass through the skin - care!
    • The other elements are faily common in a variety of minerals e.g. limestone, CaCO3, dolomite, CaCO3.MgCO3 and carnallite, KCl.MgCl(6H2O). Less common are strontianite, SrCO3, and barytes, BaCO3.
    • Radium, the bottom member of the group is radioactive with a half-life (226Ra)of about 1600 years as a a-emitter. It was first isolated from uranium ore, pitchblende, by M. and P. Curie, by laborious frational cyrstallizations.
    • Atomic radii are smaller than those group 1, while there are two valence electrons. Therefore, the elements are harder, with higher melting and vaporizing points. The enthalpies are correspondingly higher.
    • The ionic radii of the M2+ ions are much smaller than group 1 M+ ions so that hydration and lattice energies compensate for the higher ionization enthalpies needed to reach the M2+ state.
    • The beryllium ion is too small to form many ionic compounds, and magnesium ion is also quite small so they are treated separately.
    • Calcium, Strontium and Barium are a "well-behaved" group of three elements with a nice gradual change in properties.

    Beryllium

    Beryllium is obtained by the reduction of BeCl2 by Ca or Mg or the Mg reduction of BeF2. It is a very light metal, unreactive with air or water at ordinary temperatures. It dissolves in strong non-complexing acids (except HNO3 which passivates it) to give the [Be(H2O)4]2+ ion. Be also dissolves in strong aqueous bases like NaOH to evolve hydrogn and yield the beryllate, Be(OH)42- ion. Its amphoteric behaviour is similar to aluminum. Beryllium salts, which usually come hydrated, are acidic in solution due to hydrolysis:

    [Be(H2O)]2+ + H2O alt [Be(H2O)3(OH)]+ + H3O+

    Beryllium compounds are fairly covalent and the chemistry is dominated by the Be attempting to obtain an octet. There are two important strategies identified:

    1. The simple Beryllium compounds can react with Lewis bases forming, for example, BeCl2(OEt2)2, and complex anions are also formed e.g. BeF42-, and [Be(H2O)4]2+.
    2. Polymerization occurs through bridging groups to give chain polymers, e.g. (BeF2)n, (BeCl2)n and (Be(CH3)2)n. In the halides, the bridges are not electron deficient, but in the alkyls, three-centre, two-electron bonds must be invoked. alt For bulkier groups, for example certain alkoxides, the chain lengths can be reduced, or polymerization can even be inhibited, e.g. (Be(OBut)2)3 and Be(OC6H4(But)2)2 (below): alt In the gas phase the polymers break down so that the chloride is linear BeCl2. By rapidly cooling the vapour, short aggregates with two or three Be's can be isolated. Short chain anions such as M2[Be4Cl10] (M+ = K+, Rb+, Tl+ or NH4+)have similar structures.

    The compounds BeO and BeS have the wurtzite and blende structures respectively (both are known for ZnS - see Chapter 4) but the bonding would have to be considered quite covalent. Be(OH)2 in insoluble in water.

    "Basic beryllium acetate", Be4O(CH3CO2)6, which is soluble in non­polar solvents e.g. benzene, has a tetrahedral arrangement of Be atoms around the oxygen atom, and an acetate bridging each of the six edges of the tetrahedron (Figure 11-2).

    Magnesium

    Magnesium, a rather important metal, is mostly made from dolomite and/or seawater. Magnesium can be concentrated using both sources as follows: Dolomite is heated to give an intimate mixture, MgO.CaO, and then the magnesium is ion exchanged out using seawater:

    MgO.CaO + 2H2O alt Mg(OH)2.Ca(OH)2

    Mg(OH)2.Ca(OH)2 + Mg2+ alt 2Mg(OH)2 + Ca2+

    This works because Mg(OH)2 is much less soluble than Ca(OH)2.

    There are three common ways to get the magnesium metal itself:

    1. The "calcined" dolomite is be heated with ferrosilicon, and the magnesium distills out:
      MgO.CaO + FeSi alt Mg + ill-defined silicates of Ca and Fe
    2. A low-melting mixture of MgCl2, CaCl2 and NaCl is electrolysed. Magnesium is preferentially reduced at the cathode.
    3. Magnesium oxide is reduced with coke at 2000 oC which gives an equilibrium mixture of Mg and CO. Rapid cooling separates the metal from the gaseous CO.

    Magnesium is a silvery-white metal protected by an oxide coating. It is attacked by acids (which dissolve the oxide film first) with the evolution of H2 (even with HNO3, which usually reacts to give NO or NO2). It will react with water if its surface is amalgamated. It is used in ultra-light alloys and it is the metal at the centre of chlorophyll.

    Magnesium behaves somewhere between Be and the rest of the group 2 elements. Its hydroxide is not very water soluble, while the rest are, and it has some pretty covalent organo-compounds, the grignard reagents are the most important.

    Calcium, Strontium and Barium

    These metals are made by reduction of their halides by sodium in relatively small quantities. They are all softish and silvery when freashly cut, but they react readily with oxygen and water. Calcium is most used to make the hydride, CaH2, a useful reducing and drying agent.

    Binary Compounds

    Oxides

    • They are all white high-melting compounds with the NaCl structure.
    • The most important is probably CaO, made from CaCO3 and is a major ingredient in cement.
    • All except MgO, which can be fairly inert, produce a soluble, stongly basic hydroxide with water, and the carbonate with CO2. MgO quite is insoluble in water and is only very mildly basic. It is used as a stomach antacid ("milk of magnesia").

    Halides

    • Anhydrous calcium chloride is an important drying agent, often mixed with a little cobalt chloride which is blue when anhydrous and pink when hydrated and acts as an indicator of the condition of the dessicant. Magnesium Chloride is also hygroscopic. The affinity for water decreases down th egroup so that Sr and Ba halides are normally anyhdrous.

    Other Compounds

    • Calcium carbide, CaC2, made from CaO and carbon at high temperature can be a source of acetylene.
    • The hydrides are all ionic although MgH2 has come covalent character.

    Oxo Salts, Ions and Complexes

    • All form oxo salts (CO32-, SO42- etc). MgSO4.7H2O is "Epsom salts", a mild laxative, MgCO3 is used as an antacid. CaSO4.½H2O is known as "plaster of Paris" which sets with water to give CaSO4.2H2O, "gypsum". BaSO4 is used as an imaging material to obtain medical X-rays of the intestinal tract.
    • They become less soluble down the group.
    • In aqueous solution the ions are probably at least 6-coordinate.
    • There are some chelate complexes with oxygen ligands, the best known being [Ca(EDTA)]2- and also Capolyphospates which can be used to sequester Ca an dalso in its analysis.
    • The strong complexation of Mg by porphyrins is quite unusual since the ligating atoms are all nitrogen.

    Summary

    1. Beryllium:
      1. Forms covalent compounds almost exclusively, even with the most electronegative elements.
      2. Does not form Be2+ compounds, but readily achieves a maximum coordination number of 4 by forming complex ions e.g. BeF42- and [Be(H2O)4]2+ with essentially covalent bonding within the complex.
      3. Forms covalent organometallic compounds.
      4. Oxide and specially the hydroxide are amphoteric. (The hydroxide is water soluble.)
      5. The hydride, halides and alkoxides are oligomeric or polymeric with covalent bridges, and are easily cleaved by Lewis bases.
    2. Magnesium:
      1. Forms ionic compounds with partial covalent character.
      2. Forms many compounds containing the uncomplexed Mg2+ ion and tend to be 6-coordinated by the counter ions.
      3. Forms important organometallic compounds which are mainly covalent.
      4. Oxide and especially the hydroxide are weakly basic. (The hydroxide is not water soluble.)
      5. The halides are essentially ionic.
      6. The hydride is partly covalent.
    3. Calcium, Strontium and Barium:
      1. Form only ionic substances.
      2. Form basic oxides and strongly basic hydroxides, more and more soluble down the group.
      3. Form readily hydrated halide salts, especially towards the top of the group.
      4. Hydrides are ionic.
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