Carbonylation is an extremely applicable reaction where carbon monoxide is utilised to form new ketones, aldehydes and their derivatives. The CO molecule is an excellent example of a nucleophile containing an electron sink as the carbon end is negative (the nucleophilic part) and the oxygen end is positive and will accept an electron pair when the migration occurs.
The basic scheme to be followed is thus;

From product A both aldehydes and ketones can be obtained:

Aldehydes

The first step is a reduction of the C=O - for more details of this see the Reduction section.

Ketones