Lewis Acids
Borane is unusual because it is a compound without an octet. The central boron atom has only six valence electrons. It needs one more pair of electrons to obtain an octet. The boron is a Lewis acid.
Figure 2.11.1: Borane is a Lewis acid. It can accept electrons from a donor atom. The square drawn beside the boron is used to reinforce the idea that there is a vacant site for electrons there.
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- Lewis acids are often short of a complete octet.
In the main group of the periodic table, atoms in the Group 13 column (including boron and aluminum) have three valence electrons to share in order to make bonds. Sharing these electrons with three electrons from neighbors would make three bonds, and provide six electrons, not eight, in the valence shell. Another pair of electrons must be accepted from a donor to achieve an octet.
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Figure 2.11.2: Boron, aluminum and indium are from the same column of the periodic table. All three are often Lewis acidic; they can accept electrons from donors.
- Boron, aluminum and indium compounds are often Lewis acids.
The eight-electron rule does not hold throughout the periodic table. In order to obtain noble gas configurations, some atoms may need eighteen electrons in their valence shell. For example, transition metals such as titanium often follow an eighteen-electron rule. Titanium has four valence electrons and can form four bonds in compounds such as titanium tetrakis (isopropoxide), below, or titanium tetrachloride, TiCl4. However, the titanium atom in that compound has only eight valence electrons, not eighteen. It can easily accept electrons from donors.
Figure 2.11.3: Although titanium has eight electrons in this molecule, titanium tetrakis(isopropoxide), it can accommodate up to eighteen. It is a Lewis acid. The cerium atom in cerium tris(dimethylamide) comes from a similar part of the periodic table and is also a Lewis acid.
- Transition metals such as titanium, iron and nickel may have up to eighteen electrons and can frequently accept electron pairs from Lewis bases. Transition metals are often Lewis acids.
- Lanthanides such as cerium and samarium could conceivably have up to thirty-two electrons in their valence shells! They never do. However, they are usually strong Lewis acids.
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Positive ions are often Lewis acids because they have an electrostatic attraction for electron donors. Examples include alkali and alkaline earth metals in the group IA and IIA columns. K+, Mg2+ and Ca2+ are sometimes seen as Lewis acidic sites in biology, for example. These ions are very stable forms of these elements because of their low electron ionization potentials. However, their positive charges do attract electron donors.
Figure 2.11.4. A calcium ion essentially has a noble gas configuration. Nevertheless, its positive charge can attract electrons from a donor atom.
In a similar way, “early” transition metals – those that are close to the left hand side of the periodic table, especially in groups IIIB, IVB and VB – have low ionization potentials and have high positive charges or oxidation states. For example, Sc3+, Zr4+ and V5+ are common forms of some early transition metals, and they are strong Lewis acids.
Figure 2.11.5: A few alkali, alkaline earth and transition metals that are commonly found as cations.
- Many cations such as Ca2+ or Sc3+ are good Lewis acids. Their positive charges attract electrons.
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