Which one can form complex?

Explanation

Complex formation: Complex formation generally involves transition metals due to their unique electronic properties. Transition metals have incomplete d-orbitals in their valence shells, allowing them to accept lone pairs of electrons from other molecules or ions (ligands) to form coordinate covalent bonds. These bonds lead to the formation of complexes.Alkali metals: Na, K, and Li are all Group 1 alkali metals. They lose their single valence electron readily to achieve a stable noble gas configuration and form positive ions (Na+, K+, Li+). However, they don't have empty d-orbitals to accept lone pairs from ligands, limiting their ability to form complexes.Chromium: Cr, on the other hand, is a transition metal in Group 6. It has partially filled d-orbitals (3d4 configuration) in its valence shell. These empty d-orbitals can readily accept lone pairs from ligands, forming dative bonds and stabilizing the complex.Therefore, among the options provided, only chromium possesses the necessary electronic configuration (d-orbitals) to form complexes with other molecules or ions.