Haemoglobin is a naturally occurring complex in which the central metal ion is

Explanation

Each haem group holds an iron two ion coordinated to four nitrogen atoms of the porphyrin ring, with the remaining site available to bind oxygen reversibly. Magnesium is the central ion of chlorophyll and cobalt that of vitamin B12, both of which are offered here as the parallel biological cases. Carbon monoxide is toxic because it binds the same iron site far more strongly than oxygen.