Pseudohalogen concept

The pseudohalogen concept (p. 319) might lead one to expect the existence of a cyanate analogue of cyanogen but there is little evidence for NCO-OCN, consistent with the known reluctance of oxygen to catenate. By contrast. 

Halogens are important since these elements form compounds with the vast majority of the other elements in the periodic table. By means of easy concepts such as the pseudohalogen concept, correlation between structure, bonding and properties can be predicted for unknown compounds. 

The pseudohalogen concept can be extended (i) to some special nonplanar anions such as CF3, (ii) to heavier elements (isovalence electronic exchange of O by S, Se, Te of N by P etc. in SCN , SeCN , TeCN , P(CN)2 ) " and (iii) to derivatives such as the five-membered ring anion [CS2N3] which can be obtained in the reaction of carbon disulfide and sodium azide as shown by Sommer as early as 1915. The introduction of heavier elements, however, results in weaker delocalization effects due to the formation of weaker tt-bonds. 

The pseudohalogen concept has been extended to include non-linear resonance-stabilised... 

A useful theme for cohering much of the chemistry of compounds containing the CN group is the concept of pseudohalogens, a term introduced in 1925 for certain strongly bound, univalent radicals such as CN, OCN, SCN, SeCN, (and N3, etc.). These groups can form anions X , hydracids HX, and sometimes neutral species X2. 

In summary, the concept of pseudohalogens proves useful in systematizing the chemistry of some of these nonmetallic groups, but it should never be followed blindly. 

Figure 30 shows schematically the concept of HOMO level tuning in Ir pseudohalogen complexes of the type TBA[Ir(ppy)2(CN)2] (46), TBA[Ir(ppy)2... 

These inconveniences may be partly overcome by using another representation of atom Y, for instance by introducing a pseudohalogen in a semi-empirical computation. In this case the properties of the pseudohalogen may be adjusted to mimic the actual Y atom. This approach is close to the use of a pseudopotential on atom Y, which has been proposed as an efficient solution to this problem. Nevertheless, the extraction of the pseudopotential may not be very familiar to a nonspecialist and it may be difficult to reparametrize this pseudopotential if one wants to adjust the representation of the classical atom to a nonstandard local situation. It is the reason why we looked for another solution which does not require special knowledge other than the standard concepts of quantum chemistry. 

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