Divalent electronegativities

The biochemical basis for the toxicity of mercury and mercury compounds results from its ability to form covalent bonds readily with sulfur. Prior to reaction with sulfur, however, the mercury must be metabolized to the divalent cation. When the sulfur is in the form of a sulfhydryl (— SH) group, divalent mercury replaces the hydrogen atom to form mercaptides, X—Hg— SR and Hg(SR)2, where X is an electronegative radical and R is protein (36). Sulfhydryl compounds are called mercaptans because of their ability to capture mercury. Even in low concentrations divalent mercury is capable of inactivating sulfhydryl enzymes and thus causes interference with cellular metaboHsm and function (31—34). Mercury also combines with other ligands of physiological importance such as phosphoryl, carboxyl, amide, and amine groups. It is unclear whether these latter interactions contribute to its toxicity (31,36). 

Compounds having divalent sulfur and selenium atoms bound to more electronegative elements react with alkenes to give addition products. The mechanism is similar to that in halogenation and involves of bridged cationic intermediates. 

Rare earth elements are the general term for 15 kinds of lanthanide elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Py, Ho, Er, Tm, Yb, Lu) together with Sc and Y elements. They prefer trivalent states in the complex formation, though three elements (Eu, Sm, Yb) can assume tri- and divalent stateos and Ce a tri- or tetravalent state. Their ionic radii are fairly large (1.0-1.17 A) and their electronegativities are low (1.1-1.2). In fact, the former are much larger than those of... 

Biltz-Zen trend have been discussed and represented as a function of a charge transfer atomic parameter which correlates with Pauling s electronegativity. This approach has been successfully employed for groups of binary alloys formed by the alkaline earths and the divalent rare earth elements. 

The structures of some stable stannylenes, such as several amino-, " alkoxy-, and arylthio-substituted intermediates, have been revealed by X-ray crystallography. They are monomeric crystals and the tin atom has the coordination number 2. The divalent tin in such compounds is stabilized by the effects of electronegativity of the ligand atoms and by the donation of the lone-pair electrons to the vacant 5p orbital of the tin. Although the first monomeric dialkyl- and diaryl- stannylenes in... 

Recently, Senoh, Tokuyama, and Witkop (37) have studied a metal-activated enzymatic reaction in the presence and the absence of enzyme, and have discovered that the order of effectiveness of the metals is exactly the reverse in the enzymatic and nonenzymatic processes. The reaction was O-methylation of 3,4-dihydroxybenzaldehyde. In the absence of divalent metal ions, the nonenzymatic reaction yields very predominantly the paramethylated product in neutral solution, since the p-hydroxyl group is the more electronegative. Metal complex formation... 

Germanium is disposed in the center of group 14 of the periodic table of the elements. Increased stability of divalent species, which is more pronounced for tin and lead, begins from this element. The main characteristics of these atoms, such as atomic radius, energy of ionization, electron affinity, electronegativity and other features, are presented in parallel in a review12. 

Estimates of formation enthalpies of MX2 and MX4 (X = F, Cl, Br, I, S042", C032, N03 and P043 ) for Po and element 116 [150] made on the basis of the MCDF atomic calculations confirmed the instability of the 4+ state of element 116. The chemistry of element 116 is expected to be mainly cationic an ease of formation of the divalent compounds should approach that of Be or Mn, and tetravalent compounds should be formed with the most electronegative atoms, e.g., 116F4. 

The divalent tellurium compounds with a C —Te —Y (Y = S, Se) unit in the molecule are stabilized by coordination of tellurium to an electronegative atom appropriately placed in the molecule. Formyl, pyridyl, or azo groups in an or/fto-position to the tellurium atom in phenyl tellurium compounds provide these coordination sites. X-ray structural investigations of such compounds revealed short Te—O and Te —N contacts2-4. 

In the absence of such data, plots of stability constants for isostructural metal complexes formed with a common ligand against EN values for the metal ions are still useful. As shown in Figure 2, the stability of divalent metal complexes with HCO3 and S04 is practically independent of cation electronegativity, whereas the metal complexes formed with HP04 and 003 are increasingly dependent on EN, particularly for EN >1.5. Based on the relative solubilities of their salts, EN values for C0 2- ... 

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