Oxidation number

In coordination chemistry, the oxidation number of a central atom in a coordination compound is the charge that it would have if all the ligands were removed along with the electron pairs that were shared with the central atom^[1] (in a coordination compound, all shared electron pairs are donated by the ligands).

The oxidation number is used in the nomenclature of inorganic compounds. It is represented by a Roman numeral; the plus sign is omitted for positive oxidation numbers. The oxidation number is placed either as a right superscript to the element symbol, e.g. Fe^III, or in parentheses after the name of the element, e.g. iron(III): in the latter case, there is no space between the element name and the oxidation number.

The oxidation number is usually numerically equal to the oxidation state and so the terms oxidation state and oxidation number are often used interchangeably. To be more precise, however, oxidation number is used in coordination chemistry with a slightly different meaning since the rules used for counting electrons are different: every electron belongs to the ligand, regardless of electronegativity. Also, oxidation numbers are conventionally represented with Roman numerals while oxidation states use Arabic numerals. The oxidation state can differ from the oxidation number in a few cases where the ligand atom is less electronegative than the central atom (e.g., in iridium phosphine complexes), resulting in a formal oxidation state that is different from the oxidation number.

Spectroscopic oxidation states

Although formal oxidation numbers can be helpful for classifying compounds, they are unmeasureable and their physical meaning can be ambiguous. Formal oxidation numbers require particular caution for molecules where the bonding is covalent, since the formal oxidation numbers require the heterolytic removal of ligands, which essentially denies covalency. Spectroscopic oxidation states, as defined by Jorgenson and reiterated by Wieghardt, are measureables that are bench-marked using spectroscopic and crystallographic data.^[2]

See also

• List of oxidation states of the elements
• Reduced gas

References