  Using density functional theory (DFT) calculations, we investigated the
adsorption of CO2 molecules on 3d transition metal (TM)-benzene complexes. Our
calculations show that the maximum number of CO2 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 H2
adsorption number for TM atoms including Sc or Ti atoms. We found that the
18-electron rule fails to correctly predict CO2 binding on Sc- or Ti-benzene
complexes because CO2 binding, in contrast to H2 binding, requires additional
consideration for steric hindrance due to the large bond length of CO2. We
calculated the occupation function for CO2 using the Tolman cone angle, which
shows that three CO2 molecules fully occupy the available space around Sc- and
Ti-benzene complexes. This estimation is the same maximum CO2 adsorption number
predicted by DFT calculations. Therefore, we propose that the occupation
function for CO2 using the Tolman cone angle is an efficient model for
evaluating steric hindrance of CO2 adsorption on a surface.
