The Poisonous nature of Carbon monoxide (CO) is due to its

The poisonous nature of carbon monoxide (CO) is primarily due to its ability to form a stable complex with haemoglobin, known as carboxyhaemoglobin (COHb) [2]. CO has a significantly higher affinity for haemoglobin than oxygen—estimated to be 200 to 250 times greater [1]. When inhaled, CO binds strongly to the heme iron in haemoglobin, effectively displacing oxygen and reducing the blood's oxygen-carrying capacity [3]. This binding also shifts the oxygen-haemoglobin dissociation curve to the left, which hinders the release of any remaining bound oxygen to body tissues. This dual mechanism leads to cellular hypoxia, particularly affecting oxygen-sensitive organs like the brain and heart, and can be fatal [2]. While CO can reduce metal oxides and has a specific molecular structure involving sigma and pi bonds, these properties do not account for its physiological toxicity.

Sources

1. [1] https://www.ncbi.nlm.nih.gov/books/NBK430740/
2. [2] https://www.ncbi.nlm.nih.gov/books/NBK557888/
3. [3] https://www.nature.com/articles/s42003-021-01880-1