Divalent Group 14 compounds

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). 

These are of two main types compounds of M", which for platinum have been known since the beginning of this century and commonly involve the stable [PtMes] group and compounds of the divalent metals, which were first studied by J. Chatt and co-workers in the late 1950 s and are commonly of the type [MR2L2] (L = phosphine). In the Pt" compounds the metal is always octahedrally coordinated and this is frequently achieved in interesting ways. Thus the trimethyl halides, conveniently obtained... 

The final possible mode of action for an antioxidant is as a peroxide decomposer. In the sequences that lead to photodegradation of a polymer the ready fragmentation of the hydroperoxide groups to free radicals is the important step. If this step is interfered with because the peroxide has undergone an alternative decomposition this major source of initiation is removed. The additives which act by decomposing hydroperoxide groups include compounds containing either divalent sulfur or trivalent phosphorus. The mechanism involves... 

Scheme 1 Synthesis of group 14 elements polysulfido complexes from divalent compounds bearing bulky substituents...

This interpretation of the experimental results was corroborated by ab initio calculations on the relative stabilities of a series of double bond compounds, [H2Pb=X] and their plumbylene type isomers, [frans-H-Pb-X-H] and [ciT-H-Pb-X-H] (X = O, S, Se, and Te) (vide supra) 6 The results of the theoretical calculations and experiments for lead-chalcogen double bond species are in sharp contrast to those of the other Group 14 element analogues which do not isomerize to the divalent compounds.14b... 

Reaction of Divalent Compounds of Group 14 Metals with Chalcogen Sources... 

Divalent compounds of group 14 metals, i.e., heavier analogs of carbenes, are highly reactive and readily insert into various bonds. Therefore, these species are useful for the formation of M-E (M = Si, Ge, Sn E = S, Se, Te) bonds. 

It is convenient to discuss the derivatives of the remaining heavier group 14 terphenyl elements together, since they have many features in common. This is particularly true for the divalent compounds. 

The theoretical section introduced divalent carbon(O) compounds as molecules CL2 where the ligand L is a ct donor. In principle this task can be fulfilled by various neutral group 15 compounds (N2, nitriles, amines, phosphanes, arsanes, etc.), neutral group 16 compounds (sulfides, selenides, etc.) as well as by divalent C(II) with a free pair of electrons at a carbon atom, such as isonitriles, NHCs, carbenes, CO, ylides, etc. The neutral and isolable compound C2PPh3 [14,15] may also serve as a donor L to stabilize a carbon atom. 

The apparent anomaly between mercury and the lighter elements of transition group 2. in that mercury regularly forms both univalent and divalent compounds, while zinc and cadmium do so very rarely, is partly under mm id from the observation that mercury III salts ionize even in the gaseous late to Hg.. rather than Hg Evidence for this double ion is provided by its Hainan spectral line, by the lineal CI-Hg-Hg-CI units in crystals or mercury It chloride, and by the cml of incrciirytll nitrate concentration cells The anomaly is fuitlicr removed by the obsetv.ttioii that cadmium also forms a (much less stable) diatomic ton Cdj T eg., ill Cd.-lAICL) . 

Clean-up Liquid-liquid partitioning (separatory funnel) Column chromatography Divalent metal clean-up (Pb2+, Fe2+, Cu2+) Separate the toxin from groups of compounds in the sample extract... 

Divalent compounds of group-14 elements Si to Pb where the elements E are stabilized via intramolecular mono- and bidentate chelation, shown schematically in Figure 33, have been the subject of an extensive theoretical study by Scholler et al.146. The complexes are experimentally known for E = Si, Ge, Sn and L = P, but not yet for the other elements which were calculated. Table 32 gives relevant calculated bond lengths and angles. The analysis of the bonding situation shows that the central element E is weakly coordinated by the axial E—L bonds which become somewhat stronger when... 

FIGURE 33. Schematic representation of the divalent group-14 compounds studied by Scholler et al. The calculated bond lengths and angles are given in Table 32. Reproduced by permission of Wiley-VCH from Reference 146... 

TABLE 32. Optimized bond lengths (A) and angles (deg) of divalent group-14 compounds shown in Figure 33 ... 

Structure of Simple Binary Ionic Compounds.—To be electrically neutral, every ionic compound must contain some positive and some negative ions. The simplest ones are those binary compounds that contain one positive and one negative ion. Obviously the positive ion must have lost the same number of electrons that the negative one has gained. Thus monovalent metals form such compounds with monovalent bases, divalent with divalent, and so on. In other words, this group includes compounds of Li+, Na+, K+, Rb+, Cs+, Cu+, Ag+, and Au+ with F-, C1-, Br, I- of Be++, Mg++, Ca++, Sr++, Ba++, Zn++, Cd++, Hg++, with 0—, S—, Se—, Te of B+++, A1+++, Ga+++, In+++, T1+++, with... 

XS = sulfonation, M = methylolation, MO = miscellaneous oxidation, G = grafting, P = phenolation, CM = carboxymethylation, MAC = maleic anhydride copolymerization, MCR = miscellaneous carboxyla-tion reactions, E = epichlorohydrin (also in conjunction with pheno-lated lignin), A = alkoxylation (i.e., ethylene, propylene, and butylene oxides), M+EP = modification with compounds containing unsaturated end groups ( divalent hydrocarbons ) followed by epoxidation with peroxide, MA = methacrylic acid. 

Isoelectronic with monovalent group 13 molecules are divalent molecules of group 14 elements see Germanium Inorganic Chemistry Silicon Inorganic Chemistry and Silicon Organosilicon Chemistry). The most populous class of these species are the silylenes, divalent compounds of silicon, ft is not appropriate here to provide a detailed review of silicon chemistry since such material is more expertly and comprehensively covered elsewhere. However, it is pertinent to this discussion to make some mention of silylene chemistry. ... 

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