Two new ruthenium(II) arene complexes, 2a (C24H34B10FeRuS2) and 2b (C15H26B10O2RuS2), bearing a carborane unit and other different functional groups were synthesized, and their cytostatic effects on cancerous cells were evaluated. Our observations illustrate that a structural change from a ferrocene unit to a carboxyl group could lead to high selectivity toward cancer cells and facilitate the efficient inhibition of the proliferation of target cells, indicating that the tuning of the overall properties of the ruthenium(II) arene complex by appropriate ligand tagging is critical to creating a selective antineoplastic agent.