In the coordination complex, Potassium hexacyanoferrate(II), the central atom/ion acts as,

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The coordination complex is Potassium hexacyanoferrate(II), which is K₄[Fe(CN)₆] . The central metal ion is Fe²⁺ and the ligands are CN⁻ ions. In the formation of a coordination complex, the central metal ion accepts electron pairs from the ligands to form coordinate covalent bonds. According to the Lewis acid-base theory, an electron pair acceptor is a Lewis acid. Therefore, the central atom/ion acts as a Lewis acid. Answer: Lewis acid