The surface vibrations of CH2(a), CH3CH2(a) and CF3CH2(a) groups on Ag(111) have been studied by reflection-adsorption infrared spectroscopy. Compared with the corresponding iodides, obvious C-H stretch mode-softening was observed for adsorbed CH,(a) and CH3CH2(a), but not for CF3CH2(a), which is attributed to the weakening of the charge transfer from silver to the C-H anti-bond of the alkyl fragments by the fluorine atoms. These results provide the indirect evidence that the partial charge-donation into the C-H anti-bond, instead of the formation of an agostic C-H (...) M bond, is responsible for the C-H stretch mode-softening of adsorbed hydrocarbons/alkyl fragments on transition metal surfaces. (c) 2006 Elsevier B.V. All rights reserved.