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

Abstract

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 σ-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.