Ageing a mixture of sodium molybdate, malonic acid, and tris(ethylenediamine)cobalt(iii) chloride using different synthetic routes, namely, solution-based methods at room temperature or 110 °C, and a mechanochemically accelerated vapour-assisted method, yielded the polyoxomolybdate [Co(en)3]5Na[Mo7O24(μ-Mo8O26)Mo7O24]·nH2O (1). The new polyoxomolybdate anion 1 comprised three fragments, namely, two {Mo7O24} units bridged by a {Mo8O26} unit, which were interconnected by the terminal oxygen atoms of MoO6 octahedra and represent a unique structural motif not yet described in the structurally versatile chemistry of polyoxomolybdates (POMos). The ageing reaction was found to occur via a series of intermediates, two of which were isolated and identified as the heptamolybdate coordination polymer [Co(en)3]2[NaMo7O24]Cl·nH2O (2), comprising {Mo7O24} units bridged by a sodium atom, and the heptamolybdate (H3O)[Co(en)3]2[Mo7O24]Cl·9H2O (3). An identical reaction procedure with [Co(C2O4)(en)2]+ instead of [Co(en)3]3+ yielded the orthomolybdate [Co(C2O4)(en)2]2[MoO4]·9H2O (4) and the hydrogen malonate [Co(C2O4)(en)2]C3O4H3 (5). The new polyoxomolybdate [Co(en)3]5Na[Mo7O24(μ-Mo8O26)Mo7O24]·nH2O was also examined as a catalyst for the epoxidation of cyclooctene, and was superior to both heptamolybdate and octamolybdate catalysts over 24 h. The heptamolybdate [Co(NH3)6]2[Mo7O24]·8H2O (6) was isolated as the only reaction product of sodium molybdate and hexaamminecobalt(iii) nitrate in the presence of malonic acid using solution-based methods.