Lewis Acids

 Hello, welcome to today’s lesson. I believe that this series is making a lot of sense to you. Yesterday, we showed that in order to have a substance behave as a Lewis acid, then the substance would have to possess a central atom that has an empty orbital into which it can be able to



receive an electron pair. We can never stop repeating this fact because it is the most fundamental fact that you need to know about Lewis acids.


We also showed that BF3 and AlCl3 could all act as Lewis acids because we established the fact that there is a free 2pz and 3pz orbital on the central boron and aluminum atoms respectively. This is the orbital into which the electron pair is received when the BF3 or AlCl3 acts as a Lewis acid.


Here we have shown the reaction of ammonia with BF3 and AlCl3. In either case, we can see that what is going on actually the formation of a dative or coordinate covalent bond (Refer to the earlier lesson on the formation of H3O^+ ion). The ammonia molecule has a lone pair of electrons localized on the nitrogen atom. BF3 and AlCl3 have central atoms that contains an empty orbital into which the lone pairs could be donated to form a coordinate covalent bond.


Now let me tell you the common thread that links all the central atoms in compounds that acts as Lewis acids; they all possess an OPEN SEXTET. The term open sextet implies that the atom requires two electrons to complete its octet and this is readily done by accepting an electron pair as shown in the images attached.


Transition metal ions readily act as Lewis acids and this is the foundation for the formation of the kind of compounds called coordination compounds or “complexes”. We shall deal with this kind of compounds in  detail later in the series.


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