Oxides of Carbon

There are two main forms of the oxides of Carbon: Carbon Monoxide (CO) and Carbon Dioxide (CO2).

Carbon Monoxide

This is formed when carbon is burned with a deficiency of oxygen. There is a formal triple bond thereby completing the octet around each of the atoms, and this results in formal charges of -1 for C and +1 for O, and the presence of lone electron pairs on both atoms.

Carbon monoxide is a valuable ligand for the stabilization of metals in low oxidation states, as it forms synergic bonds. The lone pair on the C atom forms a σ-bond to the metal, but the electrons in the d-orbitals on the metals are delocalized into the antibonding π-orbitals of  the CO ligand by the formation of dπ(metal)-pπ(CO) bonds. (This weakens the CO bond, and so the extent of backbonding in a complex can often be obtained from vibrational spectroscpy, which measures the wavenumber of the CO bond.)

This combination of σ-donor and π-acceptor is the definition of synergic bonding, and the main interaction is the acid-base interaction between the electron donor metal and the electron acceptor CO molecule. These interactions mean that  the high electron density on the metal in its low oxidation state is delocalized over the CO ligands, and so the metal is stabilized. Examples of metals in the 0 oxidation state are Ni(CO)4, Fe(CO)5, and Mn2(CO)10. The role of CO in bonding with transition metals is very important.

Carbon Dioxide

This contains two C=O double bonds, which are each longer and have smaller force constants than the bond in CO, reflecting the presence of a triple bond in CO. It is formed by heating carbonates, giving CO2 and the metal oxide.

Carbon dioxide is the thermodynamically most stable oxide of carbon, though in the presence of carbon an equilibrium is set up, governed by the entropic favourability of the formation of two molecules of CO(g) from one molecule of CO2(g).

Carbon dioxide is weakly acidic in aqueous solution, forming Carbonic acid, H2CO3, but in alkaline solution the basic hydrogencarbonate ion, HCO3, is formed.

Carbon Dioxide is one of the main gases contributing to the Greenhouse effect. The CO2 in the atmosphere allows visible light to pass through to the earth, but it absorbs the waves which are reradiated by the earth at longer wavelengths, resulting in a gradual warming of the earth.