Chalcogens electronegativities

There are several systematic nuclear magnetic resonance studies of the interaction between the substituents and the protons and ring atoms of five-membered heterocycles. In some 2-substituted furans, thiophenes, selenophenes, and tellurophenes there is a linear correlation between the electronegativity of the chalcogen and several of the NMR parameters.28 As there also is a good correlation between the shifts of the corresponding protons and carbons in the four heterocycles, the shifts of unknown selenophene and tellurophene derivatives can be predicted when those of thiophene are known. This is of special interest for the tellurophene derivatives, since they are difficult to synthesize. In the selenophene series, where a representative set of substituents can be introduced in the 2- as well as in the 3-position, the correlation between the H and 13C shifts and the reactivity parameters according to Swain and Lupton s two-parameter equation... 

The group of the chalcogens sulfur, selenium and tellurium is a typical triad of the more electronegative nonmetals with relatively high-ionization energies, relatively strong element-element bonds and a clear tendency to form mono-and polyatomic anions (Table 1). 

Dealkylation is the main direction in the decomposition of mixed dications 124-126. Stability of these dications strongly depends on the nature of the chalcogen and substituents.119 In general, the order of stability is consistent with the difference in electronegativities and changes as follows Te > > Se > S (Scheme 46).95 119... 

Simply using the electronegativities or IPs of the chalcogens would lead to the prediction that for a series of compounds, the O-containing compound would oxidize least readily and the Te-containing compound would be most easily oxidized (Table 7). In most cases, the oxidation data match this prediction. Only in a small number of cases does it not, and in those cases it can be attributed to the structural environment in which the chalcogen is found. 

Chalcogen Atomic radius (pm) Electronegativity" Ionization energy (kJ/mol)... 

The distortion was measured by the difference between intra- and intertriangle Mo-Mo distances in the octahedra. A cation with larger formal charge and less electronegative chalcogen atoms put more electrons to the cluster. When the cluster core is more reduced as the result of the electron transfer, the octahedron tends to contract and become more regular. 

The chemical shifts of the a-carbon atoms are chiefly determined by the electronegativity of the heteroatom. An increase in the coordination number of the chalcogen center (in chalcogenium salts, cr- and ir-telluranes) leads to a downfield shift of the a-carbon atom signals. The... 

Table 1 Covalent Radii and Electronegativities for Carbon and the Chalcogen Atoms...

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