Reaction of the dimer [(eta(5)-C5Me5)RhCl(mu(2)-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(eta(5)-C5Me5)(pta)Cl-2] and [Rh(eta(5)-C5Me5)(pta)(2)Cl] 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(eta(5)-C5Me5)(CO)(pta)] and [Rh(eta(5)-C5H5)(pta)(2)] have been prepared lfrom [Rh(eta(5)-C5Me5)(CO)(2)] and [Rh(eta(5)-C5H5)(PPh3)(2)], respectively, by reaction with pta. An in vitro evaluation of these compounds, together with [Os(eta(6)-C10H14)(pta)Cl-2] and the well-characterized antimetastasis drug [Ru(eta(6)-C10H14)(pta)Cl-2], 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 to [Ru(eta(6)-C10H14)(pta)Cl-2], viz., [Rh(eta(5)-C5Me5)(pta)Cl-2] and [Os(eta(6)-C10H14)(pta)Cl-2], demonstrated very similar cytotoxicity profiles. [Rh(eta(5)-C5Me5)(pta)Cl-2] proved significantly more cytotoxic in A549 cells and [Rh(eta(5)-C5Me5)(pta)(2)Cl] Cl 3-fold more cytotoxic in T47D cells, both relative to RAPTA-C. These data suggest that the development of organometallic anticancer drugs based on the neighboring elements to ruthenium should not be overlooked