Final answer:
Cr³⁺ cannot act as a Lewis base because it does not have lone pairs of electrons to donate, unlike CN−, NH₃, and Cl− which can donate electron pairs.Correct answer is option C.
Step-by-step explanation:
The student's question involves identifying which given species cannot act as a Lewis base. A Lewis base is a species that can donate an electron pair to form a complex ion (adduct) with a Lewis acid, which is a species that can accept an electron pair.
Among the options given, CN−, NH3, and Cl− have lone pairs of electrons and thus can donate them to form bonds with Lewis acids. However, Cr3+, which is a cation with a filled valence shell and no extra electrons to donate, cannot act as a Lewis base. It is typically a Lewis acid as it can accept electron pairs.