A series of five new mononuclear neutral gold(I) complexes containing N-heterocyclic carbenes (NHCs) was synthesized and fully characterized by spectroscopic methods. The X-ray structures of four complexes are presented. These gold(I) complexes together with four other neutral gold(I)-NHC complexes previously described were evaluated in vitro against Leishmania infantum promastigotes and axenic amastigotes. Moreover, their cytotoxicity was assessed on the murine macrophages J774A.1. Except one complex (10), eight gold(I)-NHC-Cl complexes show potent activity against the pathological relevant form of L. infantum amastigote with IC in the low micromolar and submicromolar range and five of them exhibit a SI close to 10. The lead-complex 11 displays a very high and selective activity (IC = 190 nM, SI = 40.29) and constitutes the best promising gold(I)-based drug of this series.