Why does Carbon Monoxide bind with Haemoglobin 200 times more than Oxygen?
Ubczoo has got it muddled. CO has empty pi-ANTIbonding orbitals, localised mainly on the carbon, so that it is an example of a pi-accepting ligand. It accepts electron density from the iron full dxz and dyz orbitals (if we label the axis that CO is on, z). Experimental evidence: the fact that CO binds specifically to metals in low oxidation states, and that this is accompanied by a reduction in CO stretching frequency.
