Cobalt(II), in the presence of acetate and nitrate, quantitatively adds to the manganese–cobalt oxido cubane Mn^IVCo^III₃O₄(OAc)₅(py)₃ (1) to furnish the pentametallic dangler complex Mn^IVCo^III₃Co^IIO₄(OAc)₆(NO₃)(py)₃ (2). Complex 2 is structurally reminiscent of photosystem II’s oxygen-evolving center, and is a rare example of a transition-metal “dangler” complex. Superconducting quantum interference device magnetometry and density functional theory calculations characterize 2 as having an S = 0 ground state arising from antiferromagnetic coupling between the Co^II and Mn^IV ions. At higher temperatures, an uncoupled state dominates. The voltammogram of 2 has four electrochemical events, two more than that of its parent cubane 1, suggesting that addition of the dangler increases available redox states. Structural, electrochemical, and magnetic comparisons of complexes 1 and 2 allow a better understanding of the dangler’s influence on a cubane.