The Heavier Chalcogens

Sulfur, in its reduced oxidation states, has a complex chemistry due to the formation of polysulfides and their facile interconversions. Pearson estimated the HS/HS potential as 1.08 V by a thermochemical cycle, 

Mills et al. report considerable uncertainty as to the p/fa of HS, but they suggest it may be less than 7 (218). Association as in 

From the result of Henglein and Gutierrez (162), Mills et al. estimated a value of —1.3 V for the S/SH couple, in which S apparently refers to dissolved S8 (218). If we use Pearson s result for HS and estimate the hydration free energy of atomic sulfur as 16.4 kj/mol, the value for Ar, we obtain a potential of 1.44 V for reduction of atomic aqueous sulfur to HS. 

At this time little can be said about the oxidation states between atomic sulfur and S(III). There is a report of generation of S202 by photolysis of S2032- (101), but the results have been shown to be, at least in part, spurious (40). 

S(III) is found as dithionite, S2042-, which is in homolytic equilibrium with S02 in aqueous solution as in reaction (37). 

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The accessibility of the +4 and +6 oxidation states for sulfur and, to a lesser extent, selenium gives rise to both acyclic and cyclic molecules that have no parallels in N-O chemistry. Thus there is an extensive chemistry of chalcogen diimides RN=E=NR (E = S, Se, Te) (Section 10.4). In the case of Te these unsaturated molecules form dimeric structures reflecting the increasing reluctance for the heavier chalcogens to form multiple bonds to nitrogen. The acyclic molecule N=Sp3,... 

The heavier chalcogens are more prone towards secondary interactions than sulfur. In particular, the chemistry of tellurium has numerous examples of intramolecular coordination in derivatives such as diazenes, Schiff bases, pyridines, amines, and carbonylic compounds. The oxidation state of the chalcogen is also influential sulfur(IV) centres engender stronger interactions than sulfur(II). For example, the thiazocine derivative 15.9 displays a S N distance that is markedly longer than that in the corresponding sulfoxide 15.10 (2.97 A V5. 2.75-2.83 A, respectively). ... 

Paradoxically, the most firmly established dihalides of the heavier chalcogens are the dark ruby-red P0CI2 and the purple-brown PoBr2 (Table 16.5). Both are formed by direct reaction of the elements or more conveniently by reducing P0CI4 with 8O2 and PoBt4 with H28 at 25°. 

Imido chalcogen halides of the type RNEC12 (E=S, Se, Te) provide an interesting illustration of the reluctance of the heavier chalcogens to form -N=E< double bonds. The sulfur derivatives RNSX2 (X=F, Cl) are stable, monomeric compounds. 

The discovery of cyclic imides of the heavier chalcogens is a relatively recent development. The ring systems that have been structurally characterized all have bulky substituents attached to the nitrogen atoms, viz., Se3(NR)2 (74, R= Bu, Ad),30,152 Se3(N Bu)3 (75),160 Se Bu (76)208 and Se9(N Bu)6 (77).208 The only known cyclic tellurium imide is Te3(NtBu)3.150... 

The heavier chalcogen and pnictogen analogues of many of these phosphine sulfides are also known (Table 1). 

In the cationic systems, the positive charges are delocalized over almost all atoms, even if the individual structures may be described by the Zintl concept that assigns localized positive charges to tricoordinate E atoms. It appears that the Zintl concept is better suited, yet not sufficient, to describe the structures of the heavier chalcogen elements. 

The enhanced tendency of the heavier chalcogens (S< coordination spheres, either by donor-acceptor interactions with Lewis acids or bases, or by intermolecular association, allowed to characterise crystallographically in recent years a considerable amount of supramolecular structures. 

Enium Ions of Other Group 16 Elements. Furukawaetal.316,320 have obtained enium ions of the heavier chalcogen elements stabilized by intramolecular complexation with dimethylamino groups (322). Resonances of the benzylic and methyl protons in the 1H NMR spectrum of cation 322a are shifted downfield... 

The nature of multiple bonding between germanium and the heavier chalcogens in the complexes (/74-Megtaa)GeE (E = Se, Te) is best described as an intermediate between the Ge+— E and Ge=E resonance structures. The preparation of these complexes involves the addition of the elemental chalcogen to (/74-Mestaa)Ge, which is synthesized by the metathesis of GeCl2(l,4-dioxane) and Li2[Mestaa] (Mestaa = octamethyldibenzotetraaza[14]annulene dianion). The molecular structures of both complexes are shown in Figures 5 and 610. 

In view of the preference of the tetrasilabuta-1,3-diene 139 for the s-cis form, it seemed worthwhile to examine its behavior in [4 + 2] cycloadditions of the Diels-Alder type. Since 139, like many disilenes, should behave as an electron-rich diene, we attempted to react it with various electron-poor and also with some electron-rich olefins. No reaction was detected in any case. Only in the presence of water did 139 react with quinones to furnish the unsymmetrically substituted disilenes 36 and 37 (see Section III.A). The effective shielding of the double bonds by the bulky aryl groups and, above all, the 1, 4-separation of the terminal silicon atoms of about 5.40 A appear to be responsible for these failures. Thus, it was surprising that treatment of 139 with the heavier chalcogens afforded five-membered ring compounds in a formal [4 + 1] cycloaddition (see below). 

In the past few decades, almost all of the heavier chalcogen analogues of ketones, i.e. thioketones, selenoketones, and teUuroketones, have been synthesized and characterized. Both thermodynamic and kinetic stabilization methods have been applied to stabilize these unstable double-bond species. In contrast to the doubly bonded systems between carbon and heavier chalcogens, heavier group 14 element analogues of ketones are much more reactive and unstable and hence their structures and properties have not been fully disclosed until recently. ... 

Other alkali pseudohalides may be prepared from the cyanides. Mild oxidation of M+CN in solution with lead oxides produces M+OCN. The heavier chalcogens may be directly combined (e.g. equation 11). 

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