Abstract: The electronic and geometric structures,the vibrational frequencies and the intensities of a series of rhodium carbonly hydride species were investigated by use of the ab initio calculation. It was predicted that the rhodium carbonyl hydride species containning1,2 and 3 hydrogen atoms could exist during the hydrogenation of CO or CO₂on Rh catalyst.Hatom was tending to exclude CO out of the systems when the number of Hin the system increased. In all the calculated rhodium carbonyl species, the most intensive vibrational mode,the intensity of which increased with increasing Hcoadsorbed, was caused by Rh-Hvibration other than C-O, while an obvious counterside effect usually observed in complex compound was seen in(H)₃RhCO[C_s].The redshift of CO+H₂Rh system from 2050 cm^-1to about 2000 cm^-1observed in experiments might be caused by the vibration of Rh-Hinstead of that of C-O,the intensity of which was predicted to blue shift and diminish with increasing Hnumber in the rhodium carbonyl hydride species.

Key words: hodium carbonyl hydride, Ab initio RECP, Vibrational spectrum, Geometricalstructure