Carbon and Oxygen cannot react to form an ionic compound because the two elements are non-metals. To form an ionic bond, a metal combines with a non-metal through electrostatic attraction of oppositely charged ions.
The two that won't are C and O. They will react, but not ionically. O is on the left of the Periodic table and C is more or less in the middle. They form CO carbon Monoxide and CO2 which is Carbon Dioxide. They are just not ionic.
