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Some Atypical Properties of Beryllium Compounds

This page discusses three examples of beryllium behaving differently from the rest of Group 2. In fact, there are several similarities between beryllium and aluminum in Group 3.

Beryllium chloride is covalent

Physical properties

Beryllium chloride, BeCl2, melts at 405°C and boils at 520°C. The corresponding values for magnesium chloride are 714°C and 1412°C.

Notice that the boiling point of beryllium chloride is dramatically lower than that of magnesium chloride. The higher boiling point of magnesium chloride is as expected from the strong forces between the positive and negative ions present. Because its boiling point is much lower, it follows that beryllium chloride does not contain ions—it must be covalent. However, the melting point is high for a small covalent molecule, indicating more complicated factors involved.

Reaction with water

Beryllium chloride reacts vigorously and exothermically with water, evolving hydrogen chloride gas. This is typical of covalent chlorides.

Initially, BeCl2 reacts to form hydrated beryllium ions, [Be(H2O)4]2+, and chloride ions.

These hydrated beryllium ions (named as tetraaquaberyllium) are strongly acidic. The beryllium ion at the center draws the electrons in the bonds toward itself, making the hydrogen atoms in the water even more electron deficient. If the solution is hot and concentrated (which is likely in the highly exothermic process of adding water to solid beryllium chloride), chloride ions can remove one or more of these hydrogen ions to produce hydrogen chloride gas.

In contrast, all other ionic chlorides in Group 2 dissolve in water without any obvious reaction.

The structure of beryllium chloride

As a gas

Beryllium chloride is a linear molecule. The following figure shows only the outer electrons:



As is evident from the figure, beryllium chloride is an electron-deficient compound due to its two empty orbitals at the bonding level.

As a solid

If BeCl2 adopted this same structure as a solid, the melting point would be much lower than the value above. As a very small molecule, it is expected to form weak intermolecular attractions.

In the solid phase, BeCl2 molecules polymerize to make long chains. They do this by forming coordinate bonds (dative covalent bonds) between lone pairs on chlorine atoms and adjacent beryllium atoms. The following diagram shows a simple dimer—the start of the polymerization process.



The beryllium atoms are clearly still electron deficient. The process continues. The next diagram shows the coordinate bonds using arrows, which point from the atom supplying the pair of electrons to the atom with the empty orbital.



Why isn't beryllium chloride ionic?

Beryllium has a high electronegativity compared with the rest of Group 2, and therefore attracts a bonding pair of electrons towards itself more strongly than magnesium and the rest do. For an ionic bond to form, the beryllium must give up its electrons, but it is too electronegative to do so.

Beryllium forms 4-coordinated complex ions

Some simple background

Although beryllium does not normally form a simple ion such as Be2+, it does form ions in solution. When solvated, the beryllium ion attaches to four water molecules, forming a complex ion with the formula [Be(H2O)4]2+.

The ion is said to be 4-coordinated, or to have a coordination number of 4, because there are four water molecules arranged around the central beryllium.

Many hydrated metal ions are 6-coordinated. For example, magnesium ions in solution exist as [Mg(H2O)6]2+.

The water molecules in these ions are attached to the central metal ion through coordinate bonds One of the lone pairs on each water molecule forms a bond with an empty orbital in the metal ion. Each time one of these bonds is formed, energy is released, and the ion becomes more stable. It would seem advantageous for the metal ion to form as many coordinate bonds as possibly.

Why does beryllium only attach four water molecules?

The hydration of beryllium

The problem is that there must be attachment sites for the lone pairs on the water molecules. Beryllium has the electronic configuration 1s22s2. This is illustrated below:




When beryllium forms a 2+ ion it loses the 2 electrons in the 2s orbital. That leaves the 2-level completely empty. The 2-level orbitals reorganise themselves (hybridize) to make four equal orbitals, each of which can accept a lone pair of electrons from a water molecule. In the next diagram the 1s electrons have been omitted because they are irrelevant to bonding



The oxygen atom in each water molecule has two lone pairs of electrons. Only one of them is shown to avoid cluttering the diagram.

Notice that once four water molecules have bonded in this way, there is no more space available at the bonding level. All the empty orbitals from the original beryllium ion are in use. The water molecules arrange themselves as far apart as possible, pointing toward the corners of a tetrahedron. The ion therefore has a tetrahedral shape.

The hydration of magnesium

Magnesium might be expected to behave the same way, but because it is in Period 3, there are 3d orbitals available as well as 3s and 3p.

When the magnesium ion is formed, the 3s, 3p and 3d orbitals are empty. When the ion is hydrated, it uses the 3s orbital, all three of the 3p orbitals and two of the 3d orbitals, reorganizing them to keep the water ligands as far apart as possible, as shown below.



The remaining 3d orbitals cannot be used because it is impossible to physically fit more than six water molecules around the magnesium ion; they take up too much room.

What about the other ions in Group 2?

As the ions get bigger down the group, there is less tendency to form coordinate bonds with water molecules. The ions become so big that they cannot sufficiently attract the lone pairs on the water molecules to form formal bonds; instead, the water molecules cluster more loosely around the positive ions. If they do form coordinate bonds with the water, however, they are 6-coordinated like magnesium.

Beryllium hydroxide is amphoteric

The termamphoteric means beryllium hydroxide can react with both acids and bases to form salts.

The other Group 2 hydroxides

The other Group 2 hydroxides are basic. They react with acids to form salts. For example:

\[ Ca(OH)_2 (s) + 2HCl (aq) \rightarrow CaCl_2 (aq) + 2H_2O(l)\]

Calcium hydroxide reacts with dilute hydrochloric acid to produce calcium chloride and water.

Beryllium hydroxide

As with the other Group 2 hydroxides, beryllium hydroxide reacts with acids, forming solutions of beryllium salts. For example:

\[ Be(OH)_2 (s) + H_2SO_4(aq) \rightarrow BeSO_4\, (aq) + 2H_2O\, (l)\]

However, it also reacts with bases. Beryllium hydroxide reacts with aqueous sodium hydroxide, forming a colourless solution of sodium tetrahydroxoberyllate, Na2[Be(OH)4]:

\[ Be(OH)_2 (s) + 2NaOH (aq) \rightarrow Na_2Be(OH)_4(aq)\]

The product contains the complex ion, [Be(OH)4]2-.The nomenclature comes from the following: "tetra" means four; "hydroxo" refers to the OH groups; the -ate suffix in "beryllate" indicates that the beryllium is part of a negative ion.

A simple explanation

The origin of beryllium hydroxide must be considered; it was likely formed by adding sodium hydroxide solution to a solution of a beryllium salt such as beryllium sulfate.

Beryllium ions in solution exist as hydrated ions, [Be(H2O)4]2+. The beryllium has such a strongly polarizing effect on the water molecules that hydrogen ions are very easily removed.

The sodium hydroxide solution contains hydroxide ions, which are strong bases. Adding the correct amount of sodium hydroxide solution precipitates what is known as "beryllium hydroxide," but the true reaction is more complicated.

\[ Be(H_2O)_4^{2+} (aq) + 2OH^-(aq) \rightarrow Be(H_2O)_2(OH)_2(s) + 2H_2O (l)\]

The product (other than water) is a neutral complex, covalently bonded. All that has happened to the original complex ion is that two protons have been removed from the water molecules. A precipitate of the neutral complex is formed; because of its lack of charge, there is insufficient electrostatic attraction between this neutral complex and water molecules to bring it into solution.

If an acid is added, the protons originally removed are replaced. The precipitate dissolves as the original hydrated beryllium ion is re-formed.

\[ Be(H_2O)_2(OH)_2(s) + 2H^+(aq) \rightarrow [Be(H_2O)_4]^{2+} (aq)\]

If a base is added, the additional hydroxide ions pull more protons off the water molecules to give the tetrahydroxoberyllate ion:

\[ Be(H_2O)_2(OH)_2(s) + 2OH- (aq) \rightarrow [Be(OH)_4]^{2-}(aq) + 2H_2O(l)\]

Beryllium hydroxide dissolves because the neutral complex is converted into an ion which will be sufficiently attracted to water molecules.

This does not occur with the other Group 2 hydroxides. Calcium hydroxide, for example, is truly ionic; it contains simple hydroxide ions, OH-. These react with protons from an acid to form water; in this way, the hydroxide reacts with acids. However, there is no any equivalent to the neutral complex of beryllium. Adding more hydroxide ions from a base has no effect because there is nothing with which to react.


Jim Clark (