Carbon monoxide (CO) is a colorless, odorless, and tasteless flammable gas that is slightly less dense than air. It is toxic to animals that use hemoglobin as an oxygen carrier (both invertebrate and vertebrate) when encountered in concentrations above about 35 ppm, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal biological functions. In the atmosphere, it is spatially variable and short-lived, having a role in the formation of ground-level ozone.
Carbon monoxide consists of one carbon atom and one oxygen atom, connected by a triple bond that consists of a net two pi bonds and one sigma bond. It is the simplest oxocarbon and is isoelectronic with other triply-bonded diatomic species possessing 10 valence electrons, including the cyanide anion, the nitrosonium cation, boron monofluoride and molecular nitrogen. In coordination complexes the carbon monoxide ligand is called carbonyl.