Rhenium(V) complexes containing tridentate thiosemicarbazones/thiosemicarbazides (H₂L1) derived from N-[N′,N′-dialkylamino(thiocarbonyl)]benzimidoyl chlorides with 4,4-dialkylthiosemicarbazides have been synthesized by ligand-exchange reactions starting from [ReOCl(L1)]. The chlorido ligand of [ReOCl(L1)] (4) is readily replaced and reactions with ammonium thiocyanate or potassium cyanide give [ReO(NCS)(L1)] (6) and [ReO(CN)(L1)] (7), respectively. The reaction of (NBu₄)[ReOCl₄] with H₂L1 and two equivalents of ammonium thiocyanate, however, gives in a one-pot reaction [ReO(NCS)₂(HL1)] (8), in which the pro-ligand H₂L1 is only singly deprotonated. An oxo-bridged, dimeric nitridorhenium(V) compound of the composition [{ReN(HL1)}₂O] (11) is obtained from a reaction of (NBu₄)[ReOCl₄], H₂L1 and sodium azide. The six-coordinate complexes [ReO(L1)(Ph₂btu)] (12), where HPh₂btu is N,N-diphenyl-N′-benzoylthiourea, can be obtained by treatment of [ReOCl(L1)] with HPh₂btu in the presence of NEt₃. Studies of the antiproliferative effects of the [ReOX(L1)] system (X = Cl⁻, NCS⁻ or CN⁻) on breast cancer cells show that the lability of a monodentate ligand seems to play a key role in the cytotoxic activity of the metal complexes, while the substitution of this ligand by the chelating ligand Ph₂btu⁻ completely terminates the cytotoxicity.