Page 71 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

2.1 SOLUBILITY 51

Na +

ADP +
K
Figure 2.1 Schematic diagram of the Na–K pump sitting astride a lipid bilayer. (This graphic has
been obtained from the public domain source Wikimedia Commons.)

It would be wrong, however, to give the impression that the s-block elements do not have
much of a covalent chemistry at all. Thus:

1. The chemistry of hydrogen is largely covalent. Low temperature activation of
dihydrogen (H ), traditionally the preserve of transition-metal catalysts, has recently
2
been accomplished with main-group element reagents and catalysts.
2. The smaller, least electronegative s-block atoms Li, Be, and Mg also form many
covalent compounds, most notably with carbon. (Have a look at Figure 1.4 to get a
sense of the Pauling electronegativities of these elements.) Organolithium and organo-
magnesium compounds are cornerstones of organic synthesis, where they form the
basis of some of the most reliable methods of forming carbon–carbon bonds.
3. By donating single electrons, the s-block metals promote a variety of radical reactions,
including a number of reductive coupling reactions.

With those introductory remarks, we are ready to explore some of the details of s-block
chemistry.

2.1 SOLUBILITY
One of the key roles of s-block metals in chemistry is to act as counterions for anionic
nucleophiles. Some basic ideas about the solubility of s-block metal salts should therefore
allow more informed discussions as we progress through the book.
As ionic substances, most alkali metal salts and many alkaline earth metal salts dissolve
in water. Certain di- and tri-valent anions such as carbonate, sulfate, and phosphate form
insoluble salts with alkaline earth metals; examples of such insoluble salts include CaCO ,
3
BaSO , and Ca (PO ) .
4 2
4
3
For salts of strong acids, the aqueous solutions are typically neutral, for example, NaCl,
Ba(NO ) , and so on. The analogous salts of Be and Mg, however, give rise to acidic solu-
3 2
tions. The small sizes of these cations and the resulting high surface charge densities lead
to ionization of a coordinated water, as shown below:
2+
+
[Be(H O) ] + H O [Be(H O) (OH)] + H O + (2.1)
3
2
3
2
4
2
+
2+
[Mg(H O) ] + H O [Mg(H O) (OH)] + H O + (2.2)
3
2
2
2
5
6