Polycarbonyls of Rh+ formed after interaction of CO with Rh-MFI: an FTIR spectroscopic study

Rh+ ions in Rh-MFI are able to coordinate simultaneously three CO molecules at 293 K, the Rh+(CO)(3) species formed being characterized by IR bands at 2181, 2118 and 2084 cm(-1). During evacuation at 293 K the tricarbonyls are converted to Rh+(CO)(2) geminal dicarbonyls ( 2114 and 2048 cm(-1)). The latter are quite stable and are decomposed at 673 K. Negligible amounts of Rh+-CO linear species (ca. 2114 cm(-1)) are produced during the decomposition of the dicarbonyls. At 100 K in the presence of gaseous CO the tricarbonyls can attach one more CO molecule. The proved frequencies of the Rh+(CO)(4) species obtained are at 2164 2159 and 2112 cm(-1). The results are explained by the high coordinative unsaturation of Rh+ in Rh-MFI. Experiments on CO-H2O coadsorption show formation of mixed Rh+(H2O)(x)(CO)(2) complexes (2108 and 2040 cm(-1)) and also evidence that the relatively high CO frequencies of the gem-dicarbonyls are a result of the high coordinative unsaturation of Rh+ in Rh MFI and the enhancement of the sigma-component of the Rh+-CO bond as compared to Rh+(CO)(2) species formed on oxide supports. NO replaces CO from the Rh+ cations producing Rh+(NO)(2) dinitrosyls (1881 and 1786 cm(-1)). No evidence of mixed carbonyl nitrosyl species has been found.