The oxidation state of iron in Fe(CO)₅ is

In coordination and organometallic chemistry, carbon monoxide (CO) is treated as a neutral ligand by IUPAC convention — every CO group bonded to a metal carries a formal charge contribution of 0 toward the complex's overall charge. The oxidation state of the central metal in a complex is then found from the requirement that the metal's oxidation number plus the total charge contributed by all ligands must equal the complex's overall charge.

1. Fe(CO)₅ is a neutral, uncharged molecule, so its overall charge is 0.

2. It contains five CO ligands, and each one carries a formal charge contribution of 0, so together they contribute 5 × 0 = 0 to the total charge.

3. Let x be the oxidation number of the iron atom. The charge-balance equation is: x + 0 = 0.

4. Solving this equation gives x = 0.

This is confirmed independently by the compound's systematic IUPAC name, pentacarbonyliron(0), where the (0) states the metal's oxidation state directly. The same zero-oxidation-state pattern holds for other neutral homoleptic metal carbonyls, such as Ni(CO)₄ and Cr(CO)₆, reinforcing that CO's neutral-ligand convention is general, not specific to this one compound.

Hence, the oxidation state of iron in Fe(CO)₅ is 0.