Reaction of the dimer [(η5-C5Me5)RhCl(μ2-Cl)]2 with 2 or 4 equiv of the water-soluble phosphine 1,3,5-triaza-7-phosphatricyclo[]decane (pta) affords [Rh(η5-C5Me5)(pta)Cl2] and [Rh(η5-C5Me5)(pta)2Cl]Cl, respectively. Both complexes have been characterized in solution by NMR spectroscopy and in the solid state by single-crystal X-ray diffraction, the latter as the chloride and BPh4- salts. In addition, the rhodium(I) complexes [Rh(η5-C5Me5)(CO)(pta)] and [Rh(η5-C5H5)(pta)2] have been prepared from [Rh(η5-C5Me5)(CO)2] and [Rh(η5-C5H5)(PPh3)2], respectively, by reaction with pta. An in vitro evaluation of these compounds, together with [Os(η6-C10H14)(pta)Cl2] and the well-characterized antimetastasis drug [Ru(η6-C10H14)(pta)Cl2], RAPTA-C, was undertaken using HT29 colon carcinoma, A549 lung carcinoma, and T47D breast carcinoma cells. In the HT29 cell line, the two nearest congeners