The new anion [Ru(3)Rh(CO)(13)](-) (1) has been obtained by reaction of [Rh(CO)(4)](-) ([N(PPh(3))(2)](+) or [PPh(4)](+) salt) with Ru(3)(CO)(12); [RuRh(3)(CO)(12)](-) (2) has been derived by oxidative degradation of [RuRh(4)(CO)(12)](2-). Their salts, [N(PPh(3))(2)][Ru(3)Rh(CO)(13)] (I) and [N(PPh(3))(2)][RuRh(3)(CO)(12)] (II), were characterized by single-crystal X-ray diffraction. Data for I: space group P&onemacr;, a = 9.827(2) Å, b =14.911(2) Å, c = 18.735(3) Å, alpha = 110.53(1) degrees, beta = 99.79(2) degrees, gamma = 91.85(2) degrees, R1 = 0.0342, wR2 = 0.0926 for 7257 independent reflections with I > 3sigma(I). Data for II: space group P2(1)/c, a = 14.746(3) Å, b = 21.395(4) Å, c = 16.140(4) Å, beta = 102.95(2) degrees, R1 = 0.0582, wR2 = 0.1383 for 4381 independent reflections with I > 3sigma(I). Both anions have a tetrahedral metal frame; 1 has an idealized C(3) symmetry, with the Rh atom and its unique terminal carbonyl on the 3-fold axis; nine more carbonyls are terminally bonded to the three Ru atoms, while three bridge the Ru-Rh edges. In 2, which has an idealized C(s)() symmetry, three carbonyls are terminally bound on the Ru atom, and one, one, and two CO, respectively, on Rh1, Rh2, and Rh3; five more CO bridge all edges but the Ru-Rh3 edge. It is worthy of note that subtle details of the CO ligands stereochemistry allowed the correct labeling of metal centers, otherwise indistinguishable on the basis of good quality X-ray diffraction data only. Compound 1 reacts with PPh(3), yielding the rhodium-substituted [Ru(3)Rh(CO)(12)(PPh(3))](-) (1a) ((31)P NMR; 56.9 ppm (d), J(P-)(Rh) = 188 Hz). (13)C NMR spectrum of 1 is a doublet (207 ppm, J(C-Rh) = 17.5 Hz) consistently with a fluxional behavior with complete CO scrambling, from 295 down to 170 K.