Surface reactions of well-defined Rh dicarbonyl Rh-I(CO)(2)(+) supported on highly dealuminated Y zeolites (Si:Al greater than or equal to 100) have been monitored by time-resolved FTIR spectroscopy. Rh-I(CO)(2)(+) is partly decarbonylated in an atmosphere of diluted hydrogen at 200-250 degrees C, yielding a reactive monocarbonyl with v(CO) = 2096 cm(-1). In a gas flow of pure nitrogen this monocarbonyl transforms in dinitrogen complexes of Rh attached to the zeolite framework. Two well-defined dinitrogen complexes of Rh have been identified on the surface : a bis-dinitrogen complex Rh-I(N-2)(2)(+) with v(N-2) = 2244 and 2218 cm(-1) and a mixed carbonyl-dinitrogen complex Rh-I(CO)(N-2)(+) with v(N-2) = 2252 and v(CO) = 2062 cm(-1). The composition of these surface species has been verified by ligand exchange with N-15(2) and (CO)-O-18 and a corresponding force field calculation. The force constants of the CO and the N-2 stretchings are lower and higher, respectively, in the mixed carbonyl-dinitrogen complex Rh-I(CO)(N-2)(+) compared with those of the dicarbonyl and the bis-dinitrogen species, reflecting the weaker pi-acceptor ability of N-2 compared with CO. The new complexes are stable in flowing pure nitrogen at temperatures up to 250 degrees C. They react with CO, yielding the dicarbonyl, and with H-2/N-2, forming reversibly the intermediate monocarbonyl.