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 v(s)(CO) at 2176 cm(-1) and v(as)(CO) at 2142 cm(-1). The dicarbonyl structure is proven by (CO)-C-12-(CO)-C-13 coadsorption. In excellent agreement with the theoretically expected values, the mixed complexes, Rhn+((CO)-C-12)((CO)-C-13), were characterized by v((CO)-C-12) at 2164 cm(-1) and v((CO)-C-13) 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. (c) 2005 Elsevier Inc. All rights reserved.