Pseudohalogens are polyatomic analogues of

halogens The halogens () are a group (periodic table), group in the periodic table consisting of six chemically related chemical element, elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive elements astatine (At) and ten ...

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

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

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 chemical formula, formula . Its structure is . The simplest stable carbon nitride, it is a Transparency and translucency, colorless and highly toxic gas with a pungency, pungent odor. The molecule is a ...

; pseudohalide anions, such as

cyanide In chemistry, cyanide () is an inorganic chemical compound that contains a functional group. This group, known as the cyano group, consists of a carbon atom triple-bonded to a nitrogen atom. Ionic cyanides contain the cyanide anion . This a ...

ion; inorganic acids, such as

hydrogen cyanide Hydrogen cyanide (formerly known as prussic acid) is a chemical compound with the chemical formula, formula HCN and structural formula . It is a highly toxic and flammable liquid that boiling, boils slightly above room temperature, at . HCN is ...

; as

ligand In coordination chemistry, a ligand is an ion or molecule with a 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 el ...

s in

coordination complex A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

es, such as

ferricyanide Ferricyanide is the name of the anion CN)6sup>3−. It is also called hexacyanoferrate(III) and in rare, but systematic name">systematic nomenclature, hexacyanidoferrate(III). The most common salt of this anion is potassium ferricyanide, a red ...

; 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 name of the compound is composed of a base, which includes the carbon of the , suffixed with "nitrile", so for example is called " propionitrile" (or pr ...

group. Well-known pseudohalogen functional groups include

cyanate The cyanate ion is an anion with the chemical formula . It is a resonance of three forms: (61%) ↔ (30%) ↔ (4%). Cyanate is the derived anion of isocyanic acid, H−N=C=O, and its lesser tautomer cyanic acid (a.k.a. cyanol), H−O−C� ...

thiocyanate Thiocyanates are salts containing the thiocyanate anion (also known as rhodanide or rhodanate). is the conjugate base of thiocyanic acid. Common salts include the colourless salts potassium thiocyanate and sodium thiocyanate. Mercury(II) t ...

, 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 ...

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; it has Symbol (chemistry), 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 ...

is listed for comparison): is considered to be a pseudohalogen ion due to its

disproportionation reaction In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation state. The reverse of disproportionatio ...

with alkali and the ability to form covalent bonds with hydrogen.

Examples of pseudohalogen molecules

Examples of symmetrical pseudohalogen compounds (, where Ps is a pseudohalogen) include

, thiocyanogen and

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 viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...

. Another complex symmetrical pseudohalogen compound is dicobalt octacarbonyl, . This substance can be considered as a dimer of the

hypothetical A hypothesis (: hypotheses) is a proposed explanation for a phenomenon. A scientific hypothesis must be based on observations and make a testable and reproducible prediction about reality, in a process beginning with an educated guess or tho ...

cobalt tetracarbonyl, . Examples of non-symmetrical pseudohalogen compounds (pseudohalogen halides , where Ps is a pseudohalogen and X is a

halogen The halogens () are a group in the periodic table consisting of six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive elements astatine (At) and tennessine (Ts), though some authors would ...

, or interpseudohalogens , where are two different pseudohalogens), analogous to the binary interhalogen compounds, are cyanogen halides like

cyanogen chloride Cyanogen chloride is an inorganic compound with the formula . This triatomic pseudohalogen is an easily condensed colorless gas. More commonly encountered in the laboratory is the related compound cyanogen bromide, a room-temperature solid that is ...

(),

cyanogen bromide Cyanogen bromide is the inorganic compound with the chemical formula, formula BrCN. It is a colorless solid that is widely used to modify biopolymers, fragment proteins and peptides (cuts the C-terminus of methionine), and synthesize other compo ...

(),

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 ...

(),

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 str ...

() and chlorine azide (), as well as interpseudohalogens like

dinitrogen trioxide Dinitrogen trioxide (also known as nitrous anhydride) is the inorganic compound with the formula . It is a nitrogen oxide. It forms upon mixing equal parts of nitric oxide and nitrogen dioxide and cooling the mixture below −21°C (−6°F): : + ...

(),

nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...

() and

cyanogen azide Cyanogen azide is a chemical compound with the chemical formula , or more precisely . It is an azide compound of carbon and nitrogen. It is an oily, colourless liquid at room temperature. It is a highly explosive chemical that is soluble in most ...

(). Not all combinations of interpseudohalogens and pseudohalogen halides are known to be stable (e.g. sulfanol ).

Pseudohalides

Pseudohalides form univalent anions which form

binary acid Binary acids or hydracids are certain molecular compounds in which hydrogen is bonded with one other nonmetallic element. This distinguishes them from other types of acids with more than two constituent elements. The "binary" nature of binary acid ...

s with

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

and form insoluble salts with

silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...

such as

silver cyanide Silver cyanide is the chemical compound with the formula AgCN. It is a white salt that is precipitated upon treatment of solutions containing Ag+ with cyanide, which is used in some schemes to recover silver from solution. Silver cyanide is used ...

(AgCN),

silver cyanate Silver cyanate is the cyanate salt of silver. It can be made by the reaction of potassium cyanate with silver nitrate in aqueous solution, from which it precipitates as a solid. : Alternatively, the reaction : analogous to the reaction used for ...

(AgOCN),

silver fulminate Silver fulminate (AgCNO) is the highly explosive silver salt of fulminic acid. Silver fulminate is a primary explosive, but has limited use as such due to its extreme sensitivity to impact, heat, pressure, and electricity. The compound becomes p ...

(AgCNO),

silver thiocyanate Silver thiocyanate is the silver salt of thiocyanic acid with the formula AgSCN. Silver thiocyanate appears as a white crystalline powder. It is very commonly used in the synthesis of silver nanoparticles. Additionally, studies have found silver ...

(AgSCN) and

silver azide Silver azide is the chemical compound with the chemical formula, formula . It is a silver(I) salt of hydrazoic acid. It forms a colorless crystals. Like most azides, it is a primary explosive. Structure and chemistry Silver azide can be prepared ...

(). A common complex pseudohalide is a

tetracarbonylcobaltate Cobalt tetracarbonyl hydride is an organometallic compound with the formula Hcobalt, Co(CO)4. It is a volatile, yellow liquid that forms a colorless vapor and has an intolerable odor. The compound readily decomposes upon melt and ''in absentia'' o ...

. The acid cobalt tetracarbonyl hydride is in fact quite a strong

acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...

, though its low

solubility In chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...

renders it not as strong as the true

hydrogen halide 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, astatine, or ...

. 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

hydrogen chloride The Chemical compound, compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hyd ...

HCl to hydrogen tetracarbonylcobaltate ), and they can react with metals M to form compounds like MX (compare

sodium chloride Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...

NaCl to

sodium azide Sodium azide is an inorganic compound with the formula . This colorless salt is the gas-forming component in some car airbag systems. It is used for the preparation of other azide compounds. It is highly soluble in water and is acutely poisonou ...

). Nanoclusters of

aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

(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. One example of a superatom is the cluster . Sodium atoms, when cooled from vapor, naturally condense into clusters, preferentially ...

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 fl ...

ions, forming (analogous to triiodide ) and similar compounds. This is due to the effects of metallic bonding on small scales.

References

{{Authority control Pseudohalogens">