More alloying on silica: Controlling the stoichiometry and achieving a high degree of alloying of metals at ultrasmall scales for catalysis can be difficult. Double complex salts, formed by a cation like Pd(NH ₃ ) ₄ ²⁺ and an anion like IrCl ₆ ²⁻ , should be excellent precursors but are poorly soluble and difficult to adsorb directly on metal oxide surfaces. Ding et al. show that sequentially adsorbing the cations and anions from organic solvents onto a silica surface, followed by heating in hydrogen, creates well-mixed nanoparticles, most less than 3 nanometers in diameter, for a variety of alloys. These materials were then tested as catalysts for acetylene hydrogenation to ethylene. Science , this issue p. 560