If CN^ – is a Lewis base, can it also act as a Lewis acid?
The Correct Answer and Explanation is:
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Correct Answer
No, the cyanide ion (CN⁻) cannot act as a general Lewis acid in typical chemical reactions. Its fundamental and overwhelmingly dominant chemical character is that of a Lewis base. However, in the specialized context of coordination chemistry, it can function as an electron pair acceptor in a specific type of interaction known as π-backbonding.
Explanation
A Lewis base is defined as a chemical species capable of donating a pair of electrons to form a covalent bond. The cyanide ion, with its molecular structure :C≡N:⁻, is a classic example of a strong Lewis base. The negative formal charge resides primarily on the carbon atom, which possesses a highly accessible and reactive lone pair of electrons. This lone pair is readily donated to a Lewis acid, such as a proton (H⁺) to form hydrocyanic acid (HCN) or a metal cation (like Fe²⁺) to form stable coordination complexes such as hexacyanoferrate(II), [Fe(CN)₆]⁴⁻. In these interactions, CN⁻ is unambiguously the electron pair donor.
Conversely, a Lewis acid is a species that can accept a pair of electrons. Typical Lewis acids possess an incomplete electron octet (e.g., BF₃) or have low-energy, empty atomic orbitals available to accommodate an incoming electron pair (e.g., transition metal ions). The cyanide ion does not meet these criteria. Both the carbon and nitrogen atoms have a complete valence shell of eight electrons, and there are no low-energy, vacant atomic orbitals to accept an electron pair from another base in a standard acid-base reaction.
The nuance arises in its role as a ligand in coordination chemistry. While CN⁻ first acts as a Lewis base by donating its lone pair to a metal’s empty orbital (a σ-bond), it can then exhibit Lewis acidic character. The triple bond in CN⁻ has associated empty, high-energy antibonding molecular orbitals (π* orbitals). If bonded to an electron-rich transition metal (typically one in a low oxidation state), the metal can donate electron density from its filled d-orbitals back into these empty π* orbitals of the cyanide ligand. In this specific interaction, called π-backbonding, the cyanide ion is the acceptor of electron density, thereby fulfilling the definition of a Lewis acid. This back-donation strengthens the metal-ligand bond and is crucial for the stability of many organometallic complexes.
