The surface chemistry of well-defined Ru carbonyl complexes on dealuminated Y zeolites has been studied by in situ FTIR spectroscopy coupled with on-line analysis of the gas phase. Coordinatively unsaturated Ru subcarbonyls (mono- and dicarbonyls), obtained by a controlled decarbonylation of Ru tricarbonyl on the surface, react with olefins as ethylene and propylene in the gas phase and form well-defined olefin dicarbonyl complexes. The IR wavenumbers of the dicarbonyl [Ru-II(CO)(2)](2+) are shifted from 2084 to 2017 cm(-1) in the uncoordinated dicarbonyl to 2064 and 1994 cm(-1) in the ethylene complex [Ru-II(CO)(2)(CH4)](2+) and to 2060 and 1993 cm(-1) in the propylene complex [Ru-II(CO)(2)(C3H6)](2+). The olefin carbonyl complexes are active and highly selective catalysts for the dimerization of ethylene to n-butene. At 170degreesC the selectivity to n-butene is >95%. (C) 2003 Elsevier Science B.V. All rights reserved.