Oxidation number
Encyclopedia
In coordination chemistry, the oxidation number of a central atom
in a coordination compound is the charge that it would have if all the ligand
s were removed along with the electron pair
s that were shared with the central atom. Oxidation numbers are often confused with oxidation state
s.
The oxidation number is used in the nomenclature of inorganic compounds. It is represented by a Roman numeral. The oxidation number is placed either as a right superscript to the element symbol, for example FeIII, or in parentheses after the name of the element, iron(III): in the latter case, there is no space between the element name and the oxidation number.
and so the terms oxidation state and oxidation number are often used interchangeably. 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 are given in Arabic numerals. The oxidation number of a central atom may be part of the compound's name (for example iron(II,III) oxide
); the oxidation state of atoms is not included in compound names.
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.
removal of ligand
s, which essentially denies covalency
. Spectroscopic oxidation states, as defined by Jorgenson and reiterated by Wieghardt, are measurables that are bench-marked using spectroscopic
and crystallographic
data.
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
in a coordination compound is the charge that it would have if all the ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s were removed along with the electron pair
Electron pair
In chemistry, an electron pair consists of two electrons that occupy the same orbital but have opposite spins.Because electrons are fermions, the Pauli exclusion principle forbids these particles from having exactly the same quantum numbers. Therefore the only way to occupy the same orbital, i.e....
s that were shared with the central atom. Oxidation numbers are often confused with oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
s.
The oxidation number is used in the nomenclature of inorganic compounds. It is represented by a Roman numeral. The oxidation number is placed either as a right superscript to the element symbol, for example FeIII, or in parentheses after the name of the element, iron(III): in the latter case, there is no space between the element name and the oxidation number.
Oxidation number versus oxidation state
The oxidation number is usually numerically equal to the oxidation stateOxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
and so the terms oxidation state and oxidation number are often used interchangeably. 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
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
. Also, oxidation numbers are conventionally represented with Roman numerals while oxidation states are given in Arabic numerals. The oxidation number of a central atom may be part of the compound's name (for example iron(II,III) oxide
Iron(II,III) oxide
Iron oxide is the chemical compound with formula Fe3O4. It is one of a number of iron oxides, the others being iron oxide , which is rare, and iron oxide also known as hematite. It occurs in nature as the mineral magnetite. It contains both Fe2+ and Fe3+ ions and is sometimes formulated as FeO ∙...
); the oxidation state of atoms is not included in compound names.
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
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
phosphine
Phosphine
Phosphine is the compound with the chemical formula PH3. It is a colorless, flammable, toxic gas. Pure phosphine is odourless, but technical grade samples have a highly unpleasant odor like garlic or rotting fish, due to the presence of substituted phosphine and diphosphine...
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 unmeasurable 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 heterolyticHeterolysis
In chemistry, heterolysis or heterolytic fission is chemical bond cleavage of a neutral molecule generating a cation and an anion. In this process the two electrons that make up the bond are assigned to the same fragment...
removal of ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s, which essentially denies covalency
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
. Spectroscopic oxidation states, as defined by Jorgenson and reiterated by Wieghardt, are measurables that are bench-marked using spectroscopic
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
and crystallographic
Crystallography
Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...
data.