CHARACTERIZATION OF CO IN RH-6(CO)(16) NAY CLUSTERS AND ISOLATED RH+(CO)(2)/NAY BY SOLID-STATE C-13 NMR-SPECTROSCOPY/

Solid-state NMR spectroscopy of C-13 is used to structurally character ize and quantify carbonyl groups in Rh-6(CO)(16) and Rh+(CO)(2) comple xes 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 sam e shielding parameters (sigma(150) similar to 178 ppm, CSA similar to 400 ppm, and eta similar to 0.2) but exhibit greatly different spin-la ttice relaxation rates. Bridged groups exist only in clusters and are bonded to three Rh atoms. Their shielding parameters [sigma(150) simil ar 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 br idged between two Rh atoms. The Rh-6:Rh+ ratio can be measured directl y from the MAS spectra or from the relaxation data. The residual broad ening of 150-200 Hz in the MAS spectra is due to distribution of isotr opic resonances in the sample. All CO species are immobile at room tem perature on the NMR time scale (of the order of a millisecond), wherea s slow exchange between dicarbonyls on hexarhodium clusters and isolat ed rhodium cations was found at 380 K.