Atomically　precise　metal　clusters　serve　as　a　unique　model　for　unraveling　the　intricate　mechanism　of　the　catalytic　reaction　and　exploring　the　complex　relationship　between　structure　and　activity.　Herein,　three　series　of　water-soluble　heterometallic　clusters　LnCu6,　abbreviated　as　LnCu6-AC　(Ln　=　La,　Nd,　Gd,　Er,　Yb;　HAC　=　acetic　acid),　LnCu6-IM　(Ln　=　La　and　Nd;　IM　=　Imidazole),　and　LnCu6-IDA　(Ln　=　Nd;　H2IDA　=　Iminodiacetic　acid)　are　presented,　each　featuring　a　uniform　metallic　core　stabilized　by　distinct　protected　ligands.　Crystal　structure　analysis　reveals　a　triangular　prism　topology　formed　by　six　Cu2+　ions　around　one　Ln3+　ion　in　LnCu6,　with　variations　in　Cu···Cu　distances　attributed　to　different　ligands.　Electrocatalytic　oxygen　evolution　reaction　(OER)　shows　that　these　different　LnCu6　clusters　exhibit　different　OER　activities　with　remarkable　turnover　frequency　of　135 s−1　for　NdCu6-AC,　79 s−1　for　NdCu6-IM　and　32 s−1　for　NdCu6-IDA.　Structural　analysis　and　Density　Functional　Theory　(DFT)　calculations　underscore　the　correlation　between　shorter　Cu···Cu　distances　and　improves　OER　catalytic　activity,　emphasizing　the　pivotal　role　of　active-site　distance　in　regulating　electrocatalytic　OER　activities.　These　results　provide　valuable　insights　into　the　OER　mechanism　and　contribute　to　the　design　of　efficient　homogeneous　OER　electrocatalysts.　©　2024　Wiley-VCH　GmbH.