Rhodium(II) carboxylates differ greatly in antitumor activity and toxicity depending on the properties of the carboxylate group (methoxyacetate, propionate, butyrate, etc.) involved. The solubility characteristics of rhodium(II) carboxylates correlate well with both the antitumor activity and toxicity that these compounds display. The amount of rhodium which is adsorbed by tumor cells in vitro also correlates with the partition coefficient of the rhodium(II) compounds studied. Survival and toxicity studies show rhodium(II) pentanoate to possess the highest therapeutic index against the Ehrlich ascites tumor strain and also show that lengthening the carboxylate R chain beyond the pentanoate reduces the drugs' therapeutic efficacy.

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Trifluoroacetamide reacts with Rh2(OOCCH3)4 to produce the amidato-bridged complex Rh2(ONHCCF3)4. The product is a mixture of four complexes that can be separated by liquid chromatography, with the distribution of complexes in the mixture being approximately 4% (band 1), 94% (band 2), 2% (band 3), and less than 1% (band 4). The complex in band 4 was not collected in this study because of the small amount present in the mixture. The molecular weights of the complexes in bands 1, 2, and 3 were determined by mass spectrometry and were all found to be the same, indicating they are three of the four possible geometric isomers. The visible spectra of all three isomers are similar, showing two bands shifted to higher energy relative to those of the corresponding dirhodium(II) carboxylate adducts. The 19F ÑMR spectrum of the complex in band 1 shows three CF3 resonances, suggesting this-complex is the isomer with three N atoms and one O atom bonded to one rhodium and the reverse combination bonded to the other rhodium. The crystal structure of the bis(pyridine) adduct of the complex from band 2, Rh2(ONHCCF3)4(py)2, has been determined by X-ray crystallography. The compound crystallizes in the orthorhombic space group Pnma with four formula weights in the unit cell of dimensions a = 15.523 (4) Á, b = 16.613 (6) Á, and c = 10.407 (6) A. The structure was solved by the Patterson method and refined to a final R value of 7.2%. This complex has two N and two O atoms on each rhodium cis to their own kind. A Rh-Rh bond distance 2.472 (3) Á was found, which is much longer than that in the corresponding carboxylato-bridged complex. The pyridine planes are coplanar, set 45°from the bridging ligand planes and bent slightly away from the NH groups. The average Rh-Npy distance of 2.29 Á is also significantly longer than that reported for the tetraacetato-bridged complex.