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Alkaline Earth (Group II) Trends

Summary of Alkaline Earth (Group II) Trends:

1. Beryllium is the least reactive and does not react with water even at red heat and does not react with N2. Magnesium only reacts at reasonable rate with steam, calcium and strontium readily tarnish in moist air and barium tarnishes readily.(compare to point 2 in Group I Summary)

2. The ionic character of the compounds increases down the Group. Beryllium forms highly covalent compounds generally with sp3 tetrahedral geometries, e.g. [BF4]2-, [BeCl2]infinity (infinite linear polymer). Magnesium forms more polar compounds with 6-coordination. Calcium, strontium, and barium form increasingly ionic compounds with higher coordination numbers (8 is particularly common). (compare to point 4 in Group I Summary)

3. The organometallic compounds of beryllium are covalent and rather unreactive. Magnesium forms two important series of organometallic compounds, RMgX and R2Mg, which provide convenient sources of carbanions for organic synthesis. The organometallic compounds of calcium, strontium, and barium are generally more reactive and are insoluble in organic solvents.

4. The oxides become progressively more basic down the Group. BeO and Be(OH)2 are amphoteric and react with acids and strong bases such as NaOH. MgO is basic and Mg(OH)2 is weakly basic and do not dissolve in NaOH solution. The oxides of calcium, strontium, and barium are basic and the hydroxides are strongly basic. The solubilities of the hydroxides in water follow the order: Be(OH)2 < Mg(OH)2 < Ca(OH)2 < Sr(OH)2 < Ba(OH)2

(compare to points 4 & 8 in Group I Summary)

5. BeX2 (X = F, Cl, Br, or I) are covalent polymers, which are readily hydrolyzed and are Lewis acids forming adducts BeX2L2 (L = Lewis base). Magnesium, calcium, strontium, and barium halides are essentially ionic and are soluble in water. (BeX2 behavior is new, compare to point 4 & 7 in Group I Summary)

6. BeH2 is a covalent polymer, magnesium hydride is partially ionic and the hydrides of calcium, strontium, and barium are very ionic and hydridic in their properties. (BeX2 behavior is new, compare to point 4 & 7 in Group I Summary)

7. Mg2+and Ca2+ have the greatest tendency to form complexes especially with ligands which have oxygen donor atoms. For small highly charged anions the order of stability is generally:

Mg2+ > Ca2+ > Sr2+ > Ba2+ 

but for the anions, NO3-, SO4-, and IO4- the stability order is:

Mg2+ < Ca2+ < Sr2+ < Ba2+

The most important complexes of these metals are with EDTA4-. The order of stability for this and related polydentate ligands is:

Mg2+ < Ca2+ > Sr2+ > Ba2+

The calcium complex, [Ca(EDTA)]2-, is particularly important because it is water soluble and allows EDTA to solubilize calcium carbonate. Polyphosphates, e.g P2O74- and P3O105- are able to function similarly to solubilize hard water deposits of CaCO3.

The crown polyethers and cryptate ligands also form stable complexes with Ca2+ and Mg2+.

(compare to point 9 in Group I Summary)

8. Both Mg2+ and Ca2+ have important roles in biology due to their fast to moderate ligand exchange rates; slower than K+and Na+ but faster than most transition metal cations. (compare to point 9 in Group I Summary)

9. The thermal stabilities of the nitrates, carbonates, and peroxides increase down the column. (same as point 5 in Group I Summary)

10. The solubilities of the sulfates, nitrates, and chlorides increase down the group. (compare to point 8 in Group I Summary)

11. The solubilities of the halides in alcohols increase down the group. (compare to point 8 in Group I Summary)

Anomalous nature of beryllium

1. The high enthalpy of atomization of beryllium causes it to be mechanically harder, higher melting, less dense, and less reactive than the heavier elements of the group.

2. The high charge/size ratio of Be2+ leads to compounds that are more covalent and complexes that are more stable than those of the remaining Group II cations. Many of the compounds have anomalously low melting points, enthalpies of formation, and are more soluble in organic solvents. The compounds are stronger Lewis acids. The halides are hygroscopic and fume when exposed to moist air.

3. Unlike magnesium and the heavier metals of the group, beryllium oxide and hydroxide are amphoteric.

4. Beryllium salts are much less thermally stable because the high lattice energy of the oxide lowers the Gibbs energy change for the decomposition reaction. Similarly it odes not form a peroxide or superoxide. With ethyne, beryllium forms the carbide, Be2C, rather than the ethnide presumably because the lattice energy of the carbide is very favorable.