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Ferrocene (Fc)/ferrocenium (Fc+)-decorated carbon nanotube electrode materials have shown promise for selectively adsorbing arsenic (As) over dissimilar anions like Cl– and ClO4–, and isostructural transition-metal oxyanions for water remediation; however, the competition between same-group oxyanions (such as arsenate vs phosphate) is underexplored and poorly understood. We use ab initio calculations to examine the competitive binding of As(V), P(V), and As(III) to Fc/Fc+ with and without functional substitutions (OH, SH, NH2, COOH, CH3, C2H5, NO2, and Cl). This work aims to understand factors that induce the selective binding of toxic arsenic over phosphate. We find that neat Fc cannot distinguish the three oxyanions because physical forces (electrostatics and dispersion) dominate the Fc-oxyanion interactions. However, combined oxidation and substitution effects enable selectivity for As(V) over P(V). Oxidation of Fc to Fc+ allows the formation of Fc+-oxyanion covalent bonds with varying donor–acceptor character depending on the oxyanion. Additionally, NH2 and SH groups that donate charge to the base Fc+ molecule and H-bond to oxyanion induce an energetic preference for As(V) over P(V) by −0.23 and −0.13 eV, respectively. Differences in pKa between As(V)/P(V) and As(III) preclude any preference for As(III) over the other anions. Using the calculated energetics, we predict the pH-dependent binding selectivity of functionalized ferrocenium. These findings demonstrate the challenges of Fc/Fc+-oxyanion interaction for selective binding and provide a path for identifying other molecules and substituents for efficient metallocene adsorbent design.