New type of rhodium gem-dicarbonyls formed in Rh-ZSM-5: An FTIR spectroscopy study

CO adsorption on Rh-ZSM-5 leads to the formation of both the well-known Rh+(CO)2 gem-dicarbonyls and another kind of dicarbonyl species as yet unknown. The latter are most probably formed with the participation of Rhn+ (n>1) ions in cationic positions in the zeolite. These species are characterized by νs(CO) at 2176 cm−1 and νas(CO) at 2142 cm−1. The dicarbonyl structure is proven by 12CO-13CO coadsorption. In excellent agreement with the theoretically expected values, the mixed complexes, Rhn+(12CO)(13CO), were characterized by ν(12CO) at 2164 cm−1 and ν(13CO) at 2106 cm−1. In contrast to the classical gem-dicarbonyls, the new species are destroyed in the presence of water. This is consistent with the proposed higher oxidation state of rhodium in this case.