Which of the following elements can form complexes?

Complex formation is a characteristic property of transition elements. These elements have partially filled d-orbitals, which allow them to accept electron pairs from ligands and form coordinate covalent bonds. A complex compound consists of a central metal ion surrounded by molecules or ions called ligands that donate electron pairs to the metal center.

Among the given elements — sodium (Na), lithium (Li), potassium (K), and chromium (Cr) — only chromium belongs to the transition metal series. It has the electronic configuration [Ar] 3d⁵ 4s¹, meaning it contains partially filled d-orbitals. This property enables chromium to form a wide variety of coordination complexes such as [Cr(H₂O)₆]³⁺, [Cr(CN)₆]³⁻, and [Cr(NH₃)₆]³⁺.

In these complexes, the ligands like water (H₂O), ammonia (NH₃), or cyanide (CN⁻) donate lone pairs of electrons to the vacant d-orbitals of chromium, forming stable coordinate bonds. The ability to form complexes is also linked to chromium’s variable oxidation states (+2, +3, +6), which allow it to bond with different ligands under various conditions.

On the other hand, sodium, lithium, and potassium are alkali metals belonging to Group 1 of the periodic table. They have a stable electron configuration after losing one valence electron and do not possess vacant low-energy orbitals suitable for complex formation. Hence, they rarely form stable coordination compounds.

Chromium’s capacity to form complexes is an important reason for its wide use in electroplating, pigments, and catalysis. The study of such coordination compounds is a fundamental concept in inorganic chemistry, especially under transition metal chemistry