a, Representation of two tungsten-based WOCs ({Ru4W20}27 and {Co4W18}20). b, Representation of the new cluster family with mixed Mo/W addenda positions. c, Representation of two molybdenum-based WOCs ({CoMo6}26 and {Co2Mo10}26). Red spheres, oxygen; purple spheres, cobalt; brown spheres, ruthenium; orange spheres, phosphorous; grey spheres, silicon. Teal polyhedra, {WO6} units; yellow polyhedra, {MoO6} units; mixed teal–yellow polyhedra, {MO6} units, where M = Mo/W.