Using density functional theory (DFT) calculations, we investigated the adsorption of CO 2 molecules on 3d transition metal (TM)-benzene complexes. Our calculations show that the maximum number of CO 2 molecules adsorbable on Sc or Ti atoms is three, but the 18-electron rule predicts it should be four. The 18-electron rule is generally successful in predicting the maximum H 2 adsorption number for TM atoms including Sc or Ti atoms. We found that the 18-electron rule fails to correctly predict CO 2 binding on Sc-or Ti-benzene complexes because CO 2 binding, in contrast to H 2 binding, requires additional consideration for steric hindrance due to the large bond length of CO 2 . We calculated the occupation function for CO 2 using the Tolman cone angle, which shows that three CO 2 molecules fully occupy the available space around Sc-and Ti-benzene complexes. This estimation is the same maximum CO 2 adsorption number predicted by DFT calculations. Therefore, we propose that the occupation function for CO 2 using the Tolman cone angle is an efficient model for evaluating steric hindrance of CO 2 adsorption on a surface.