Identify the Lewis acid and the Lewis base, and then write the product (a complex) for the following Lewis acid-base reactions: (a) \(\mathrm{PF}_{5}+\mathbf{F}^{-} \rightarrow\) ? (b) \(\mathrm{SO}_{2}+\mathrm{Cl}^{-} \rightarrow\) ?

When identifying Lewis acids in chemical reactions, we look for species that can accept an electron pair. These are typically molecules or ions that have incomplete octets, vacant d orbitals, or positive charges. For instance, in the reaction
\(\mathrm{PF}_{5} + \mathbf{F}^{-} \rightarrow ?\),
\(\mathrm{PF}_{5}\) is able to expand its valence shell beyond eight electrons, which makes it an ideal candidate to be a Lewis acid. Its structure has a phosphorus atom that can form more bonds, accepting an electron pair from a Lewis base to fill its expanded octet. Thus, understanding the electronic structure and the nature of atoms involved can assist in identifying Lewis acids in various reactions.

Lewis bases are the opposite of acids in this type of reaction; they are species ready to donate an electron pair. A classic indicator of a Lewis base is a negative charge, suggesting an excess of electrons. This is evident in the reaction
\(\mathrm{PF}_{5} + \mathbf{F}^{-} \rightarrow ?\),
where the \(\mathbf{F}^{-}\) ion, having an extra electron, acts as the Lewis base. Not exclusively ions, molecules with lone pairs of electrons, such as ammonia (NH3), water (H2O), or alcohols (R-OH), are also typical Lewis bases. In identifying a Lewis base, look for atoms with lone pairs that are not involved in bonding, as these are often the donors in Lewis acid-base reactions.

Complex ion formation is a central feature of Lewis acid-base reactions. A complex ion is a species consisting of a central metal ion bonded to one or more ligands, which are molecules or ions that donate an electron pair to the metal, forming coordinate covalent bonds. From the reactions provided, for example,
\(\mathrm{PF}_{5} + \mathbf{F}^{-} \rightarrow \textrm{PF}_{6}^{-}\),
the result is a complex ion where \(\textrm{F}^{-}\) becomes a ligand to the \(\textrm{PF}_{5}\) Lewis acid. Such complex ions are often found in transition metal chemistry where the metals act as Lewis acids, forming intricate structures with various ligands.

At the heart of a Lewis acid-base reaction is electron pair donation. The Lewis base provides a pair of electrons, while the Lewis acid accepts this pair, leading to the formation of a coordinate covalent bond. In the given examples,
\(\mathrm{PF}_{5} + \mathbf{F}^{-} \rightarrow \textrm{PF}_{6}^{-}\) and \(\mathrm{SO}_{2} + \mathrm{Cl}^{-} \rightarrow \textrm{SO}_{2Cl}^{-}\),
we see this donation clearly. The fluoride ion (\textbf{F}-) donates an electron pair to \(\textrm{PF}_{5}\), and the chloride ion (\textbf{Cl}-) to \(\textrm{SO}_{2}\). The ability to donate an electron pair is a key quality of a Lewis base, which enables the formation of new chemical species, enriched by the bonds formed during these interactions.