Pseudohalogens are polyatomic analogues of

halogens The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this group i ...

, whose chemistry, resembling that of the true

, allows them to substitute for halogens in several classes of

chemical compounds A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

. Pseudohalogens occur in pseudohalogen molecules,

inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemist ...

molecules A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...

of the general forms ''Ps''–''Ps'' or ''Ps''–X (where ''Ps'' is a pseudohalogen group), such as

cyanogen Cyanogen is the chemical compound with the formula ( C N)2. It is a colorless and highly toxic gas with a pungent odor. The molecule is a pseudohalogen. Cyanogen molecules consist of two CN groups – analogous to diatomic halogen molec ...

; pseudohalide anions, such as

cyanide Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...

ion; inorganic acids, such as

hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on a ...

; as

ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elect ...

s in

coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as '' ligands'' or complexing agents. M ...

es, such as ferouscyanide; and as functional groups in organic molecules, such as the

nitrile In organic chemistry, a nitrile is any organic compound that has a functional group. The prefix '' cyano-'' is used interchangeably with the term ''nitrile'' in industrial literature. Nitriles are found in many useful compounds, including me ...

group. Well-known pseudohalogen functional groups include

cyanate Cyanate is an anion with the structural formula , usually written . It also refers to any salt containing it, such as ammonium cyanate. It is an isomer of the much less stable fulminate anion .William R. Martin and David W. Ball (2019): "Sma ...

thiocyanate Thiocyanate (also known as rhodanide) is the anion . It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Mercury(II) thiocyanate was formerly used in pyr ...

, and

azide In chemistry, azide is a linear, polyatomic anion with the formula and structure . It is the conjugate base of hydrazoic acid . Organic azides are organic compounds with the formula , containing the azide functional group. The dominant appli ...

Common pseudohalogens and their nomenclature

Many pseudohalogens are known by specialized common names according to where they occur in a compound. Well-known ones include (the true halogen

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

is listed for comparison): Au⁻ is considered to be a pseudohalogen ion due to its disproportionation reaction with alkali and the ability to form covalent bonds with hydrogen.

Examples of pseudohalogen molecules

Examples of symmetrical pseudohalogens (''Ps''–''Ps'') include

(CN)₂, thiocyanogen (SCN)₂,

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3 ...

H₂O₂. Another complex symmetrical pseudohalogen is dicobalt octacarbonyl, Co₂(CO)₈. This substance can be considered as a

dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...

of the

hypothetical A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obser ...

cobalt tetracarbonyl, Co(CO)₄. Examples of non-symmetrical pseudohalogens (''Ps''–X), analogous to the binary interhalogen compounds, are cyanogen halides like ClCN or BrCN;

nitryl fluoride Nitryl fluoride, NO2F, is a colourless gas and strong oxidizing agent, which is used as a fluorinating agent and has been proposed as an oxidiser in rocket propellants (though never flown). It is a molecular species, not ionic, consistent with it ...

, and other compounds. Sometimes

nitrosyl chloride Nitrosyl chloride is the chemical compound with the formula NOCl. It is a yellow gas that is commonly encountered as a component of aqua regia, a mixture of 3 parts concentrated hydrochloric acid and 1 part of concentrated nitric acid. It is a ...

NOCl also is considered as pseudohalogen. Not all combinations are known to be stable.

Pseudohalides

Pseudohalides are the univalent anions (or functional groups) which form hydracids with hydrogen and form insoluble salts with

silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...

such as

s, fulminate,

and

. A common complex pseudohalide is a tetracarbonylcobaltate . The acid HCo(CO)₄ is in fact quite a strong

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a se ...

, though its low

solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...

renders it not as strong as the true

hydrohalic acids In chemistry, hydrogen halides (hydrohalic acids when in the aqueous phase) are diatomic, inorganic compounds that function as Arrhenius acids. The formula is HX where X is one of the halogens: fluorine, chlorine, bromine, iodine, or astatine. ...

. The behavior and chemical properties of the above pseudohalides are identical to that of the true halide ions. The presence of the internal multiple bonds does not appear to affect their chemical behavior. For example, they can form strong acids of the type HX (compare HCl to HCo(CO)₄), and they can react with metals to form compounds like MX (compare NaCl to NaN₃). Nanoclusters of

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

(often referred to as

superatom In chemistry, a superatom is any cluster of atoms that seem to exhibit some of the properties of elemental atoms. Sodium atoms, when cooled from vapor, naturally condense into clusters, preferentially containing a magic number of atoms (2, 8, 20 ...

s) are sometimes considered to be pseudohalides since they, too, behave chemically as

halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a flu ...

ions, forming Al₁₃I₂⁻ (analogous to I₃⁻) and similar compounds. This is due to the effects of

metallic bonding Metallic bonding is a type of chemical bonding that arises from the electrostatic attractive force between conduction electrons (in the form of an electron cloud of delocalized electrons) and positively charged metal ions. It may be des ...

on small scales.

References

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