IV Compounds

In the series of the binary halides of selenium and tellurium, the crystal structure determinations of tellurium tetrafluoride (100) and of tellurium tetrachloride on twinned crystals (65, 66) were the key to understanding the various and partly contradictory spectroscopic and other macroscopic properties (e.g., 66,161,168,169,219,220, 412), as well as the synthetic potential of the compounds. In contrast to the monomeric molecular i//-tbp gas phase structures with C2v symmetry (417), the solid state structures of both are polynuclear. As the prototype of the chlorides and bromides of selenium and tellurium, crystalline tellurium(IV) chloride has a cubane-like tetrameric structure with approximate Td symmetry (Fig. 1). Within the distorted TeCla+a octa-hedra the bonds to the triply bridging chlorine ligands are much longer than to the terminal chlorines. The bonding system can be described either covalently as Te4Cli6 molecules, or, in an ionic approximation, as [(TeCl Cn4] with a certain degree of stereochemical activity of the lone pairs toward the center of the voluminous cubane center (65, 66). 

In the tellurium(IV) fluoride structure with chains of corner-sharing TeFs tetragonal pyramids (100), the lone pair (E) is stereochemically much more active and occupies one position in the (/(-octahedral TeFsE polyhedra, in accordance with simple VSEPR considerations (Fig. 2). 

A similar trend is observed as a function of the central chalcogen atoms in the series from sulfur (electronegative and hard) to selenium and tellurium (less electronegative and softer) as central atoms, the symmetry of the XY polyhedra in the solid compounds increases (i.e., the stereochemical activity of the inert pairs decreases) and the tendency to form ionic substructures such as is greatly reduced. 

More examples for these trends, which have been discussed somewhat more generally 222, 438), will be shown in the following. 

Significant progress has been reported in the last years on all the known chalcogen(IV) halides in their structure and bonding they all are related to the three prototype molecular structures referred to above. Results in detail are reviewed here. 

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The oxidation state -1-4 is predominantly covalent and the stability of compounds with this oxidation state generally decreases with increasing atomic size (Figure 8.1). It is the most stable oxidation state for silicon, germanium and tin, but for lead the oxidation state +4 is found to be less stable than oxidation state +2 and hence lead(IV) compounds have oxidising properties (for example, see p. 194). 

Group IV. Compounds insoluble in water, but soluble in dilute hydrochloric acid. 

Organotelluriumfll and IV) compounds undergo transmetallation with Pd(II)[414], The carbonylation of the alkenylphenyltellurium(II) 459 gives the a,/3-Unsaturated ester 460 and benzoate, 460 being the main product[415], Reductive coupling of diaryl, dialkyl, and aryl alkyltellurides 461 to give 462 proceeds by treatment with Pd(OAc)2[416,417],... 

Tin, having valence of +2 and +4, forms staimous (tin(II)) compounds and stannic (tin(IV)) compounds. Tin compounds include inorganic tin(II) and tin(IV) compounds complex stannites, MSnX., and staimates, M2SnX, and coordination complexes, organic tin salts where the tin is not bonded through carbon, and organotin compounds, which contain one-to-four carbon atoms bonded direcdy to tin. 

Peroxidic Compounds. When hydrogen peroxide is added to a solution of titanium(IV) compounds, an intense, stable, yellow solution is obtained, which forms the basis of a sensitive method for determining small amounts of titanium. The color probably results from the peroxo complex [Ti(02)(0H)(H20)J, and crystalline salts such as K2[Ti(02)(S0 2] H20 can be isolated from alkaline solutions. The peroxo ligand is bidentate the two oxygen atoms ate equidistant from the titanium (98). 

The six coordinated titanium(IV) compounds, Ti(acac)2(X)2, where X is methoxy, ethoxy, isopropoxy, -butoxy, or chloro, all adopt the cis-configuration. This is beheved to result from the ligand-to-metal TT-electron donation (88,89). 

The extraction of osmium(VI) and osmium(IV) compounds with dimerkaptotiopiron by polar solvents has been resear ched. 50 -100 - multiple concentrating has been carried out in acid solutions regardless from chemical form of Os(VI) and Os(IV) with the following determination of each of them from one sample by amperemetric titration of reextracts by means of DT. A methodic of atom absor ption determination of osmium has been suggested. 

In contrast to the behaviour of 9.4a, the acyclic Te(IV) compound Cl2Te(NSO)2 can be isolated from the reaction of 9.4c with CI2 under mild conditions.Subsequent reaction of Cl2Te(NSO)2 with CI2 generates the bicyclic compound Cl6Tc2N2S (Section 8.10). 

Because of the lower stability of Se(IV) compounds as compared with their Te(IV) analogues (95UK527), the reaction of the disilyldiimine with SeCU ends up in phenanthro[9,10-c]-l,2,5-selenadiazole. 3,4-(2,4-Di-terr-butylbenzo)-l,2,5-telluradiazole 75 was obtained by a treatment of the tellurium diimide 76 with a lithium salt of tris(rerr-butyl)aniline. Dimer 77 is an intermediate in this reaction (96IC9). 

Titanium-IV compounds with their Lewis acid activity may catalyze an interfering rearrangement of the starting allylic alcohol or the epoxy alcohol formed. In order to avoid such side-reactions, the epoxidation is usually carried out at room temperature or below. 

Platin-oxyd, n. platinum oxide, specif, platinic oxide, platinum(IV) oxide. -oxydul, n. platinous oxide, platinum(II) oxide, -oxy-dulverbindung, /. platinous compoimd, platinum (II) compound, -oxydverbindung, /. platinic compound, specif, platinum (IV) compound. 

Plumbi-. plumbic, plumbi-, lead(lV). -oxyd, n. plumbic oxide, lead dioxide, lead(IV) oxide, -salz, n. plumbic salt, lead(IV) salt, -verbindimg, /. plumbic compoimd, lead(IV) compound. 

Stanni-ozyd, n, stannic oxide, tin(IV) oxide, -reihe, /. stannic series, tin(IV) series, -salz, n. stannic salt, tin(IV) salt, -sulfo-zyanid, n. stannic thiocyanate, tin(IV) thiocyanate. -verbindung, /, stannic compound, tin(IV) compound,... 

Lead forms two types of chemical compounds lead (II), and lead (IV) compounds based on Pb24 and Pb4 ions, where those based on Pb2 ions are the more stable. The metal is oxidized even at room temperature to lead oxide (PbO) and also by water that contains oxygen and forms lead hydroxide (Pb(OH),). In the lead-acid battery, the (less stable) lead (IV) oxide (lead dioxide, Pb02), is of greatest importance. Beside these two, a number of oxides are observed in the battery that are mostly mixtures. A brief survey will now be given of those compounds that are of interest for lead-acid batteries. 

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