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[3.3.1.1]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