What are the Pseudohalogens or halogenoids?

We know that dimers of halide ions are called halogens, In the similar way covalent dimers of the pseudohalide ions are called pseudohalogens or halogenoids. 

The pseudohalogens known are :

Cyanogen (CN)2

Oxycyanogen (OCN)2

Thiocyanogen (SCN)2

Selenocyanogen (SeCN)2

Tellurocyanogen (TeCN)2

Azido carbon disulphide (SCSN3)2

What are Pseudohalides ? and What is similarity with halides?

The ions consisting of two or more electronegative atoms of which at least one is nitrogen, that have properties similar to those of halide ions. These ions are called pseudohalide ions, Pseudohalide ions are univalent and these form salts resembling halide salts. The pseudohalide ions are :
Cyanide ions (CN^-)  
Isocyanide ion (NC^-)
Cyanate ion (OCN^-) 
Fulminate ion (ONC^-)
Thiocyanate ions(SCN^-)
Isothiocyanate ion (NCS^-)
Selenocyanate ion (SeCN^-)
Tellurocyanate ion (TeCN^-)
Azide ion (N3-) : 
Azido carbon disulphide ion (SCSN^-3)

The best known pseudohalide is CN-.This resembles Cl- , Br- and I- in the following respects:
(i) It forms an acid, HCN. 
(ii) It can be oxidised to (CN)₂ molecule. 
(iii) It forms insoluble salts with Ag+, Pb²⁺ and Hg₂²⁺
(iv) It forms large number of complexes similar to halide complexes, e.g. [Cu (CN)₄]^2- and [CuCl₄]₂, [Co(CN)₆]^3- and [CoCl₆]^3- 
(v) Inter-pseudohalogen compounds ClCN, BrCN and ICN can be formed. 
(vi) AgCN is insoluble in water but soluble in ammonia likeAgCl.

Related Questions:

Why halogens are insoluble in water ?

Is F2 bond is stronger than Br2 ?

Which is best known pseudohalide ?

Iodine forms I3- but F2 does not form F3- ions. Why?

Number of lone pair present at central atom of I3- ion and also give hybridisation and shape of ion?

How many maximum atoms of IF7 are present in the same plane ?

What are the Pseudohalogens or halogenoids?

What are Pseudohalides ? and What is similarity with halides?

Although electron affinity of chlorine is higher than fluorine, yet fluorine act as better oxidizing agent than chlorine? Explain.

Although electron affinity of chlorine is higher than fluorine, yet fluorine act as better oxidizing agent than chlorine? Explain.

Why chlorine trifluoride (ClF3) exists but fluorine trichloride (FCl3) does not?

Why Chlorine (Cl2) bleaches a substance permanently but sulphuredioxide (SO2) does it temporarily?

What is the correct order of electronegativity of Cl atoms in HClO, HClO2, HClO3 and HClO4 ?

How does the thermal stability of oxy acids of chlorine increase with an increase in the oxidation state of chlorine atom ?

Which of the following is a strongest acid among oxy acids of chlorine and why? "HClO, HClO2, HClO3, HClO4"

Which will have very fast tendency to dimerized ClO2 or NO and why?

Which of the following is a strongest acid among oxy acids of chlorine and why? "HClO, HClO2, HClO3, HClO4"

What is Steric Inhibition of Deprotonation effect ? Or SID effect ?

We know that picric acid is weak acid than expected this is fact that when picric acid releases a proton it becomes an anion for which repulsion results between oxygen atoms of phenoxide ion and oxygen of nitrate group, commonly known as Steric Inhibition of Deprotonation (SID effect). This also results in slight unstability of phenoxide ion and also decrease acidity. Thus picric acid is a weak acid than expected ever.

Related Questions:

1. Highly branched carboxylic acids are less acidic than unbranched acids. Explain why?
2. Benzamide is less easily hydrolysed than methyl benzoate. Why ?
3. Alcohols and Phenols both are acidic but phenols are more acidic. Why?
4. Which is more acidic 2-methyl benzoic acid , 3-methyl benzoic acid or 4-methyl benzoic acid.?
5. Why is o-toluic acid (2-methyl benzoic acid) stronger than benzoic acid ?
6. What is the ortho effect? Why are nearly all ortho substitute benzoic acids (except some groups) stronger acid than benzoic acid?
7. Name of groups which does not show ortho effect in benzoic acid?
8. Why Picric acid (2,4,6-trinitrophenol) is weak acid then expected ?
9. What is Steric Inhibition of Deprotonation effect ? Or SID effect ?
10. Alcohols and Phenols both are acidic but phenols are more acidic. Why?
11. Among orthochlorophenol and orthofluorophenol, which will be a stronger acid and why?
12. Why The phenoxide ion is more stable than phenol ?
13. What is correct acidic strengths order of the haloforms acids ? Give correct explanation.
14. Lower aliphatic acids having 1 to 5 carbons are soluble in water but higher fatty acids having six or more than six carbon are insoluble in water but soluble in alcohol. Explain.
15. Phenoxide ion is more stable than an alkaoxide. why?
16. Why does aqueous sodium bicarbonate solution dissolve carboxylic acids but not phenol though they are also acidic ?
17. Chloroform is more acidic than fluoroform why?
18. Fluorine is more electronegative than chlorine even then, p-flurobenzoic acid is weaker acid than p-chlorobenzoic acid explain ?

Why Picric acid (2,4,6-trinitrophenol) is weak acid then expected ?

Picric acid is weak acid than expected due to the presence of three Nitro groups (show strong -I as well as -M effect); this is fact that  6-membered cyclic H-bonding takes place between H of -OH and O atoms of -NO2 groups which lower acidity significantly than the expected acidity.

And second fact that when picric acid releases a proton it becomes an anion for which repulsion results between oxygen atoms of phenoxide ion and oxygen of nitrate group, commonly known as Steric Inhibition of Deprotonation (SID effect). This also results in slight unstability of phenoxide ion and also decrease acidity. Thus picric acid is a weak acid than expected ever.

Name of groups which does not show ortho effect in benzoic acid?

Mostly groups are show Ortho effect, however for groups like -NH₂ or -OH does not experience SIR effect due to small size and –CN is linear group so SIR effect not applicable.