# Why is CO_2 a lewis acid?

Still the best way of making a $C - C$ bond is to pour a Grignard reagent onto dry ice as shown...
$R - M g X + C {O}_{2} \left(s\right) \stackrel{\text{dry ether}}{\rightarrow} R C {O}_{2}^{-} + M g {X}_{2} \left(s\right) \downarrow$
The carboxylate ion can be reprotonated by water work-up to give $R C {O}_{2} H$... And here carbon dioxide has accepted the formal pair of electrons conceived to be localized on the Grignard reagent...