<p>    </p> <p>Surface functionalization of metallic nanoparticles is an attractive route to tailor the ensemble geometry and redox properties of active sites in heterogeneous catalysts. However, it is challenging to generate well-defined interfaces through conventional impregnation and one-pot colloidal synthesis methods. In this work, we utilize ligand exchange reactions for post synthetic surface modification of colloidal nanoparticles to generate unique core-shell and surface alloy structures. We use halometallate and metal chalcogenide complexes to create surface sites that are active for electrocatalytic hydrogen evolution reaction (HER). </p> <p>We synthesize a self-limiting monolayer of metal chalcogenides on colloidal Au nanoparticles through biphasic ligand exchange reaction between ammonium tetrathiomolybdate (NH₄)₂MoS₄ complex and Au nanoparticles. Through a combination of spectroscopy techniques and computational methods, we show that strong Au-S interactions introduce electronic and geometric distortion to the geometry and bond metrics of MoS₄^2-complex. Moreover, proximal MoS₄ units adsorbed on the Au surface interlink to form small MoSx oligomers with highly active bridging disulfide sites. Consequently, these core-shell AuMoS₄ nanoparticles exhibit significantly higher HER activity than MoS₄^2- supported on non-interacting carbon supports under highly acidic electrolyte conditions. Although post catalysis characterization reveals partial hydrolysis of surface adsorbed MoSx species, stable HER activity under bulk electrolysis condition indicates that active sites remain persistent. </p> <p>In an effort to extend these ligand exchange reactions to create metal/metal interfaces on other coinage metal nanoparticles such as Ag, we design metal-ligand coordination complexes to mitigate undesired galvanic replacement reactions. By varying the strength and number of coordinating ligands, we fine-tune the redox potential of oxidized noble metal precursors and confine the deposition of noble metals to a few surface layers of the Ag nanoparticles. We utilize organic amine and phosphine ligands to generate Ag@AgM core-shell nanoparticles, where M = Pd, Pt, and Au. Surface alloy or pure metal shells of Pd and Pt on Ag nanoparticles generated through this ligand-based strategy exhibited high precious metal atom utilization in electrocatalytic hydrogen evolution reaction. </p>