The　major　bottlenecks　for　ammonia　(NH3)　synthesis　under　mild　conditions　are　the　activation　of　the　extremely　inert　N　N　bond　(941　kJ　mol(-1))　and/or　the　desorption　of　NH3　from　the　catalyst　surface.　Electron　donation　from　the　appropriate　promoters　is　essential　to　enhance　N-2　activation　over　Fe　or　Ru　catalysts.　Nevertheless,　despite　typical　element　promoters　enhancing　the　N-2　activation　efficiency,　they　induce　strong　NH3　binding　on　the　catalyst　surface,　leading　to　the　need　for　high　temperatures　and　pressures　for　the　reaction　to　complete.　Herein,　we　propose　the　use　of　a　molecular　promoter　(C-60)　to　tackle　the　difficulties.　The　positioning　role　of　C-60　at　a　1　nm　scale　on　small　Ru　nanoclusters　drives　the　exposure　of　more　terrace　sites　(geometric　effect)　and　induces　d-pi　interactions　at　the　Ru-C-60　junctions.　The　latter　electronically　modifies　the　Ru　sites　(electronic　effect),　thereby　synergistically　contributing　to　N-2　and　H-2　activation　as　well　as　to　the　release　of　NH3　on　the　Ru　sites.　The　significant　electron　buffer　attribute　of　C-60　and　the　strong　electronic　interaction　between　Ru　and　C-60　facilitate　a　shift　of　d-band　center　toward　the　Fermi　level　and　a　decrease　of　work　function,　simultaneously　satisfying　the　electronic　requirements　for　N-2　activation　enhancement　and　NH3　binding　weakening.　Consequently,　the　C-60-promoted　Ru/LaN　catalyst　exhibits　a　high　NH3　synthesis　rate.　It　is　envisioned　that　our　findings　have　significant　implications　for　the　rational　search　of　molecular　promoters　for　high-efficiency　NH3　synthesis.