INTRODUCTION

We had studied the chemical properties of alkaline earth metals. We learned about the reactions of the alkaline earth metals with oxygen, water and acids or also studied the thermal stability of their oxy salts. We must contain noticed that the 1^st member of the alkaline earth metal always behave differently from the rest. We will be studying the anomalous behaviour of beryllium and as well the complexation behaviour of the alkaline earth metals in this chapter.

Complex-forming behaviour of alkaline earth metals:

A significant property of the alkaline earth metals is their complex behaviour. As we have seen previous, complex formation is favoured via small, highly charged cations with suitable empty orbitals of approximately the right energy with that the Ligand orbitals can join. Alkaline earth metal form more complexes as compared to alkaline metals. The tendency to form complex (mostly with O & N donors) decreases by increasing atomic number. Therefore, of the heavier ions, only Ca²⁺ forms a complex through ethanol. Beryllium having the smallest ion in the group tends to form complex most willingly it mostly forms complexes with tetrahedral arrangement since of the available orbitals as revealed below.                               

Figure: below shows the tetrahedral structure of hetrafluoroberyllates.

Figure: Tetrafluoroberyllates complex ion (BeF4)^2-  

The arrows in figure above indicate that two F^- ions from coordinate bonds with BeF₂. Though, once such are formed, all the Be-F bonds tend to become similar.

Beryllium forms white crystalline molecular oxide - carboxylates of which basic beryllium acetate viz [OBe₄ (CH₃COO)₆] is typical. It is utilized for the purification of Be since of its volatility and solubility in organic solvents. Beryllium forms a number of chelates by legends as oxalates, [Be (C₂O₄)₂]^2- and B-diketonate anions.

Fig: Beryllium Oxalate Complex, ion, [Be(C₂O₄)₂ ]^2-  

In the hydrated salt, instance BeCO₃.4H₂O and BeSO₄.4H₂O beryllium ions exist in the form [Be (H₂O₄)]²⁺ wherever they illustrate a coordination number of 4. Magnesium is recognized to form a extremely significant complex occurring in nature as in chlorophyll; a green pigment of plants that produces sugar in the presence of sunlight, carbon dioxide and water in a process recognized as photosynthesis.

                      Chlorophyll

6CO₂ + 6H₂O      →         C₆H₁₂O₆ + 6O₂

                       Sunlight                                glucose

Magnesium in chlorophyll is coordinated by four nitrogen atoms in the heterocyclic porphyrin ring system.

Fig: Skeleton of chlorophyll II molecule

The rest of the alkaline earth elements from calcium to barium form complexes only by strong complexation agent these as acetylacetone, ethylene diaminetetra acetic acid [EDTA] etc. In fact titrations are performed using EDTA in buffer solution to estimate the amount of Ca²⁺ and mg²⁺   present in water to determine the hardness of water.

Anomalous nature of beryllium:

Beryllium, the 1^st member of the alkaline earth metal group, differs from the members of the group just as lithium differs from 3 members of its group. In facts beryllium differs more from magnesium than lithium does from sodium. Theanomalous nature of the 1^st member of s- and p-block groups becomes more pronounced toward the middle of the table. Beryllium as well illustrates a diagonal resemblance to aluminium in similar way as lithium does to magnesium.

The properties in that beryllium differ from magnesium it shares with aluminium in common. Let us now look at such properties one at a time.

A. The cohesive properties of beryllium are much greater than those of magnesium and other elements in the group. As a result, it is much harder and has higher melting points.

B. It has smaller atomic radii higher electron affinity and higher ionization energy.

C.  Its higher polarizing power leads to all its compounds being largely covalent with lower melting and boiling points, and enthalpies of formation (example BeF₂, m.p 1073 K whereas for the rest of the group is about 1573 k). All the compound of Be are more soluble in organic solvents than the analogous magnesium compounds. They hydrolyse in water and in this respect they rather resemble aluminium.

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