Carbon Monoxide
Properties
| State | Gas at room temperature |
| Color | Colorless |
| Solubility | Slightly soluble in water (2.3 mL/100 mL at 20 °C) |
| Melting Point | -205 °C |
| Boiling Point | -191.5 °C |
About Carbon Monoxide
Carbon monoxide is one of the more chemically interesting small molecules in the periodic table. The C–O bond is a formal triple bond — the strongest of any neutral diatomic at 1072 kJ/mol — with a small dipole moment of only 0.122 D pointing the wrong way (carbon is the negative end, despite oxygen being more electronegative) because the lone pair on carbon dominates the dipole calculation. That carbon lone pair is what makes CO the most common ligand in transition-metal carbonyl chemistry: it σ-donates from carbon and π-back-accepts into empty C–O π* orbitals from filled metal d orbitals, which is why Ni(CO)4, Fe(CO)5, Cr(CO)6, and Mn2(CO)10 exist at all. The same back-bonding lengthens the C–O bond and shifts the IR carbonyl stretch from 2143 cm⁻¹ in free CO down into the 1900 to 2100 cm⁻¹ range in metal complexes — a shift any inorganic chemist reads off an IR spectrum to gauge the electron density at the metal. Industrially, CO is half of synthesis gas (syngas, CO + H2) made by steam reforming methane or partial oxidation of coal, and feeds the Fischer–Tropsch process for synthetic fuels, methanol synthesis (CO + 2H2 → CH3OH at 250 °C, 50 to 100 atm over Cu/ZnO/Al2O3), and the Monsanto/Cativa acetic acid process (CH3OH + CO → CH3COOH over Rh or Ir catalysts with iodide promotion). In metallurgy, CO is the workhorse reductant of the blast furnace: 3 CO + Fe2O3 → 2 Fe + 3 CO2, with the CO regenerated continuously from coke. The medical hazard is well known: CO binds hemoglobin with about 230 times the affinity of O2, locking heme into the carboxyhemoglobin form and starving tissues of oxygen, with symptoms beginning at 10 percent COHb and lethality near 50 percent. At nanomolar concentrations CO is also an endogenous gasotransmitter, produced by heme oxygenase, regulating vasodilation and inflammation alongside NO and H2S.
Where you'll encounter it
Every IR spectrum of a metal carbonyl complex starts with a glance at the carbonyl stretch — the position of that band, between roughly 1850 and 2100 cm⁻¹, tells you in seconds how electron-rich the metal center is. In a synthesis lab, CO under pressure runs hydroformylation (the oxo process), Pauson–Khand cyclizations, and carbonylative cross-couplings, generally introduced from a lecture bottle into a Schlenk line with explicit safety protocols. Domestically, the danger is unvented combustion appliances and idling cars in attached garages — UL-listed CO detectors alarm at 70 ppm sustained for one to four hours, which corresponds to roughly 10 percent COHb in a sedentary adult. Steel mills run on it whether they describe it that way or not.
Common Uses
- Reductant in the iron blast furnace, converting Fe2O3 to metallic Fe
- Half of synthesis gas (CO + H2) for methanol, Fischer–Tropsch fuels, and oxo aldehydes
- Carbonylation feedstock for the Monsanto/Cativa acetic acid process
- Ligand in transition-metal carbonyl chemistry — Ni(CO)4, Fe(CO)5, Cr(CO)6, Mn2(CO)10
- Endogenous gasotransmitter regulating vascular tone via heme oxygenase at nanomolar levels
Safety Information
CO is colorless, odorless, and tasteless — no sensory warning at all, which is why detectors are mandatory wherever combustion equipment runs. OSHA PEL is 50 ppm (8-hour TWA), NIOSH REL 35 ppm with a 200 ppm ceiling, IDLH 1,200 ppm. Symptoms scale with carboxyhemoglobin: headache and impaired judgment near 10 percent COHb, confusion and chest pain at 30 percent, unconsciousness at 40 percent, death above 50 percent. The standard treatment is 100 percent O2 by mask (drops the half-life of COHb from about 5 hours on room air to 1 hour) or hyperbaric O2 at 2.5 to 3 atm (about 20 minutes). Also flammable in air between 12.5 and 74 percent; explosive in the right mixture with H2 — a real concern in Ni(CO)4 production. Treat any building with a sealed-combustion heater or attached garage as requiring a UL 2034 CO detector.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.