Pseudohalogen

The pseudohalogen concept was introduced by Lothar Birckenbach [de] and Karl Kellermann [wd] in 1925.^[2][3] They defined the word pseudohalogen in order to describe a pattern of halogen-like behavior while avoiding the disputed term radical; it is unclear whether this term had reached its modern sense at the time.^{[4]: 12901 [5]: 786} They initially applied it to describe the chalcocyanate (cyanate OCN, thiocyanate SCN, selenocyanate SeCN, tellurocyanate TeCN), azide (N₃), and cyanide (CN) groups.^[3][5]

Cotton and Wilkinson (1968) give the following criteria for Ps to be an ideal pseudohalogen group:^[6]

• Ps₂ is a volatile, covalently bonded molecular substance, which is symmetrical (structure Ps−Ps). The Ps group must contain more than one electronegative atom; interpseudohalogens Ps−Ps' must contain more than two such atoms.
• Ps₂ reacts with metals M to yield salts Mⁿ⁺(Ps⁻)_n that contain Ps⁻ anions, analogous to the ionic halides Mⁿ⁺(X⁻)_n
• Ps⁻ reacts with oxidants to re-form Ps₂
• Ps also forms covalent pseudohalides APs_n analogous to the covalent halides AX_n
• In particular, Ps forms covalent compounds Ps−X and Ps−Ps' with other halogens X and pseudohalogens Ps' that are analogous to the interhalogens X−X'
• HPs is an acid
• The salts Mⁿ⁺(Ps⁻)_n are insoluble for Mⁿ⁺ = Ag⁺, Hg2+2, Pb²⁺
• Ps⁻ forms similar metal complexes to the halogens, such as HgPs2−4, the pseudohalogen analog of tetrachloromercurate(II)

Downs and Adams (1973) provide a similar list of criteria, with additional comments as follows:^[2]

• Ps₂ reversibly undergoes alkali-induced disproportionation: Ps₂ + 2OH⁻ ⇌ Ps⁻ + OPs⁻ + H₂O
• Ps₂ adds across double bonds: Ps₂ + CH₂=CH₂ → PsCH₂CH₂Ps
• Ambident pseudohalogens, such as thiocyanate, may form multiple isomeric compounds with other groups, such as methyl thiocyanate CH₃−SCN and methyl isothiocyanate CH₃−NCS
• Some pseudohalogens form trimeric anions analogous to the polyhalides X−3, such as (SeCN)−3 and [I(SCN)₂]⁻
• Hydrogen pseudohalides (HPs) are typically weaker acids than the hydrogen halides
• Ag⁺, Hg2+2, Pb²⁺ pseudohalide salts are often sparingly soluble
• Pseudohalogen–metal complexes often have different stability constants to the analogous halogen–metal complexes

Not all these criteria need be met for Ps to be considered a pseudohalogen. For example, the parent compound (−OCN)₂ of cyanate, a classical pseudohalogen group, has never been prepared.^[2][6] Conversely, if the class of pseudohalogen groups is widened to include any univalent group possessing several of the properties above, it becomes extremely broad, encompassing groups such as nitryl (NO₂), thiyl (RS), hydroxy (HO), fluorosulfate (FSO₃), and perchloryl (ClO₃).^[2]

Cotton and Wilkinson consider the most important pseudohalides to be cyanide, thiocyanate, selenocyanate, azide, cyanate, and CS₂N−3 (whose structure was later determined to be that of the substituted heterocycle 1,2,3,4-thiatriazole-5-thiolate, (−S−C(−S⁻)=N−N=N−)).^[6][7] Golub and Köhler [de] (1978) instead list azide, cyanide, fulminate CNO⁻, cyanate, thiocyanate, selenocyanate, dicyanamide N(CN)−2, and tricyanomethanide C(CN)−3.^[8][9]: 15

Examples of symmetrical pseudohalogen compounds (Ps−Ps, where Ps is a pseudohalogen) include cyanogen (CN)₂, thiocyanogen (SCN)₂ and hydrogen peroxide H₂O₂. Another complex symmetrical pseudohalogen compound is dicobalt octacarbonyl, Co₂(CO)₈. This substance can be considered as a dimer of the hypothetical cobalt tetracarbonyl, Co(CO)₄.

Examples of non-symmetrical pseudohalogen compounds (pseudohalogen halides Ps−X, where Ps is a pseudohalogen and X is a halogen, or interpseudohalogens Ps¹−Ps², where Ps¹ and Ps² are two different pseudohalogens), analogous to the binary interhalogen compounds, are cyanogen halides like cyanogen chloride (Cl−CN), cyanogen bromide (Br−CN), nitryl fluoride (F−NO₂), nitrosyl chloride (Cl−NO) and chlorine azide (Cl−N₃), as well as interpseudohalogens like dinitrogen trioxide (O=N−NO₂), nitric acid (HO−NO₂) and cyanogen azide (N₃−CN).

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

Pseudohalides form univalent anions which form binary acids with hydrogen and form insoluble salts with silver such as silver cyanide (AgCN), silver cyanate (AgOCN), silver fulminate (AgCNO), silver thiocyanate (AgSCN) and silver azide (AgN₃).

A common complex pseudohalide is a tetracarbonylcobaltate [Co(CO)₄]⁻. The acid cobalt tetracarbonyl hydride HCo(CO)₄ is in fact quite a strong acid, though its low solubility renders it not as strong as the true hydrogen halide.

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 HCl to hydrogen tetracarbonylcobaltate HCo(CO)₄), and they can react with metals M to form compounds like MX (compare sodium chloride NaCl to sodium azide NaN₃).

In addition to cyanate and thiocyanate, a range of other chalcogen and pnictogen analogs have been prepared or studied theoretically.^[10][11]

Many non-symmetric pseudohalides can bond covalently to give two isomeric products. For example, the conjugate acids of cyanate are cyanic acid (H–OCN) and isocyanic acid (H–NCO), which are tautomers of each other. The thiocyanate complexes [(NH₃)₅Co-SCN]²⁺ and [(NH₃)₅Co-NCS]²⁺ are linkage isomers of each other that are distinctly differently colored.^[12]

Many pseudohalogens are known by specialized common names according to where they occur in a compound. The true halogen chlorine is listed for comparison.

More information Group, Dimer ...

Group	Dimer	Hydrogen compound	Pseudohalide anion	Ligand name	In organic compounds	Formula	Structural formula	Ref
True halogens
chloro	chlorine	hydrogen chloride	chloride	chlorido- chloro-	-yl chloride	~ Cl	−Cl
Classical pseudohalogen groups
cyano	cyanogen	hydrogen cyanide	cyanide	cyanido- cyano-	-nitrile -yl cyanide	~ CN	−C≡N	[2][6]
isocyano	diisocyanogen⁠[13]	hydrogen isocyanide		isocyanido- isocyano-	-yl isocyanide	~ NC	−N+≡C−
cyanate	never isolated[14]	cyanic acid	cyanate	cyanato-	-yl cyanate	~ OCN	−O−C≡N	[2][6]
isocyanate	diisooxocyan [wd]⁠[14]	isocyanic acid		isocyanato-	-yl isocyanate	~ NCO	−N=C=O	[2][6]
thiocyanate	thiocyanogen	thiocyanic acid	thiocyanate	thiocyanato-	-yl thiocyanate	~ SCN	−S−C≡N	[2][6]
isothiocyanate	isothiocyanogen [wd]⁠[15]	isothiocyanic acid		isothiocyanato-	-yl isothiocyanate	~ NCS	−N=C=S	[2][6]
selenocyanate	selenocyanogen [wd]⁠[16]	selenocyanic acid	selenocyanate	selenocyanato-	-yl selenocyanate	~ SeCN	−Se−C≡N	[2][6]
isoselenocyanate	isoselenocyanogen	isoselenocyanic acid		isoselenocyanato-	-yl isoselenocyanate	~ NCSe	−N=C=Se	[17][18]
tellurocyanate[19]	tellurocyanogen	tellurocyanic acid	tellurocyanate	tellurocyanato-	-yl tellurocyanate	~ TeCN	−Te−C≡N	[2]
isotellurocyanate	isotellurocyanogen	isotellurocyanic acid		isotellurocyanato-	-yl isotellurocyanate	~ NCTe	−N=C=Te	[18]
azide	hexanitrogen	hydrazoic acid	azide	azido-	-yl azide	~ N3	−N−−N+≡N ↕ −N=N+=N−	[2][6]
Other groups sometimes considered to be pseudohalogens
fulminate	cyanogen di-N-oxide [wd]⁠[20]	fulminic acid	fulminate	fulminato-	-nitrile oxide⁠[a]	~ CNO	−C≡N+−O−	[9]: 15
dicyanamide	never isolated⁠[22]	never isolated⁠[9]: 434	dicyanamide	dicyanamido-	-yl dicyanamide⁠[b]	~ N(CN)2	···[NCNCN]−···	[9]: 15
tricyanomethanide	hexacyanoethane	tricyanomethane (cyanoform)	tricyanomethanide	tricyanomethanido-	-yl tricyanomethanide	~ C(CN)3	−C(−C≡N)3	[9]: 15
1,2,3,4-thiatriazol-5-thiolate	bis(1,2,3,4-thiatriazol-5-yl)disulfane [wd]	2H-thiatriazole-5-thione [wd]⁠[c]	1,2,3,4-thiatriazol-5-thiolate	1,2,3,4-thiatriazol-5-thiolato-	-yl 1,2,3,4-thiatriazol-5-thiolate	~ CS2N3		[6][9]: 466 [25]
trinitromethanide	hexanitroethane	trinitromethane (nitroform)	trinitromethanide	trinitromethanido-	-yl trinitromethanide	~ C(NO2)3	−C(−NO2)3	[9]: 467
cobalt carbonyl	dicobalt octacarbonyl	cobalt tetracarbonyl hydride	tetracarbonylcobaltate	tetracarbonylcobaltato-	-	~ Co(CO)4	−Co(C≡O)4	[9]: 468
cyapho	cyaphogen	phosphaethyne	cyaphide	cyaphido- cyapho-	-yl cyaphide	~ CP	−C≡P	[26]
phosphaethynolate		never isolated[27]	phosphaethynolate	phosphaethynolato-	-yl oxyphosphaalkyne	~ OCP	−O−C≡P	[28]
phosphaketenyl	never isolated[28][d]	phosphaketene[27]		phosphaketenyl-	-yl phosphaketene[e]	~ PCO	−P=C=O	[28]
hydroxyl	hydrogen peroxide (dioxidane)	water (oxidane)	hydroxide	hydroxido- hydroxy-	-ol	~ OH	−O−H
sulfanyl	hydrogen disulfide (disulfane)	hydrogen sulfide (sulfane)	hydrosulfide	sulfanido- thiolato-	-thiol -yl mercaptane	~ SH	−S−H
selanyl	hydrogen diselenide (diselane)	hydrogen selenide (selane)	hydroselenide	selanido- selenolato-	-selenol	~ SeH	−Se−H
tellanyl	hydrogen ditelluride (ditellane)	hydrogen telluride (tellane)	hydrotelluride	tellanido- tellurolato-	-tellurol	~ TeH	−Te−H
nitric oxide	dinitrogen dioxide	nitroxyl	nitroxide	nitrosyl	nitroso-	~ NO	−N=O
nitrogen dioxide	dinitrogen tetroxide	nitrous acid	nitrite	nitryl	nitro-	~ NO2	−NO2

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