Solid-state NMR spectroscopy of C-13 is used to structurally characterize and quantify carbonyl groups in Rh-6(CO)(16) and Rh+(CO)(2) complexes entrapped in NaY zeolite. MAS NMR line shape simulations show that the dicarbonyl groups present in Rh-6(CO)16 and Rh+(CO)2 have the same shielding parameters (sigma(150) similar to 178 ppm, CSA similar to 400 ppm, and eta similar to 0.2) but exhibit greatly different spin-lattice relaxation rates. Bridged groups exist only in clusters and are bonded to three Rh atoms. Their shielding parameters [sigma(150) similar to 251 ppm, CSA similar to 180 ppm, and negligible anisotropy (eta)] differ from bulk Rh-6(CO)(16) and from those found earlier for CO bridged between two Rh atoms. The Rh-6:Rh+ ratio can be measured directly from the MAS spectra or from the relaxation data. The residual broadening of 150-200 Hz in the MAS spectra is due to distribution of isotropic resonances in the sample. All CO species are immobile at room temperature on the NMR time scale (of the order of a millisecond), whereas slow exchange between dicarbonyls on hexarhodium clusters and isolated rhodium cations was found at 380 K.