Metal-oxo clusters are commonly isolated from water with ligands to prevent precipitation of metal hydroxides. However, unligated clusters are important for both mechanistic studies and applications. Open-shell transition metal clusters are especially difficult to isolate without ligation. Herein, we elucidate a cluster isolation process that yields an unligated Zn2+-Al3+-Cr3+ polyoxocation. For decades, Cr3+-polyoxocations have proved elusive synthetic targets due to hydrolytic instability. We overcame the synthetic challenges by the following strategies: 1) pH-driven hydrolysis by oxidative dissolution of zinc, 2) metal nitrate concentrations 10× higher than conventional syntheses, suppressing cluster assembly, 3) azeotropic evaporation of HNO3-H2O driving cluster-assembly and crystallization at the solution-surface. Contrary to common cluster growth, the fully assembled cluster is never detected in the reaction solution. Because these reactive clusters do not persist in solution, uncontrolled precipitation of metal hydroxide is avoided. The proposed formation pathway opens opportunities to expand the composition space of metal-oxo clusters.