Nucleophilically attacking CO2

The formation of carbonic acid in water can be considered an example of this:

Attacking CO₂ with an organometallic carbanion gives a carboxylate salt, which can be protonated to give a carboxylic acid - a useful way of creating of adding an extra carbon to a molecule.

It is usually done practically by adding dry ice to the organometallic (dissolved in an ether). It can also be done by bubbling dry CO₂ through the solution.

Overall it is a three-step process: