Oxidation states, lead compounds

A comparison of the structural chemistry of these two elements reveals some interesting resemblances and also some remarkable differences. The atoms of each element have the same outer electronic structure, two s and two p electrons, and each has oxidation states of 2 and 4. The more metallic nature of lead is shown by the difference between the structures of the elements (Chapter 29) and by many differences between stannous and plumbous compounds. It will be convenient to deal first with Sn(iv) and Pb(iv) since the structural chemistry is more straightforward for the higher oxidation state. Few compounds of Sn(ii) and Pb(ii) are isostructural and compounds containing these metals in both oxidation states are different for the two elements (e.g. Sn2S3 and Pb304). 

High oxidation state leads to covalent compounds and low oxidation state leads to ionic compounds. Therefore, CrO is ionic and basic and C1O3 is covalent and acidic. 

Because lead tends to be in the +2 oxidation state, lead(IV) compounds tend to undergo reduction to compounds of lead(II) and are therefore good oxidizing agents. A case in point is Pb02- In Chapter 19, we noted its use as the cathode in lead-add storage cells. The reduction of Pb02(s) can be represented by the half-equation... 

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). 

The concept of oxidation states is best applied only to germanium, tin and lead, for the chemistry of carbon and silicon is almost wholly defined in terms of covalency with the carbon and silicon atoms sharing all their four outer quantum level electrons. These are often tetrahedrally arranged around the central atom. There are compounds of carbon in which the valency appears to be less than... 

Lead, like tin, forms only one hydride, plumbane. This hydride is very unstable, dissociating into lead and hydrogen with great rapidity. It has not been possible to analyse it rigorously or determine any of its physical properties, but it is probably PbH4. Although this hydride is unstable, some of its derivatives are stable thus, for example, tetraethyllead, Pb(C2Hj)4, is one of the most stable compounds with lead in a formal oxidation state of + 4. It is used as an antiknock in petrol. 

Variable oxidation state is also exhibited in the oxides themselves among metals in this region of electronegativity. Thus lead, for example, forms the monoxide PbO (+2) and the dioxide PbO 2 ( + 4) (the compound Pbj04 is not a simple oxide but is sometimes called a compound oxide). Similarly, manganese gives the oxides MnO and Mn02-... 

Lead forms two series of compounds corresponding to the oxidation states of +2 and +4. The +2 state is the more common. Compounds of lead(IV) are regarded as covalent, those of lead(II) as primarily ionic. Lead is amphoteric, forming plumbous (Pb(II)) and plumbic (Pb(IV)) salts as well as plumbites and plumbates, respectively. 

In general, the chemistry of inorganic lead compounds is similar to that of the alkaline-earth elements. Thus the carbonate, nitrate, and sulfate of lead are isomorphous with the corresponding compounds of calcium, barium, and strontium. In addition, many inorganic lead compounds possess two or more crystalline forms having different properties. For example, the oxides and the sulfide of bivalent lead are frequendy colored as a result of their state of crystallisation. Pure, tetragonal a-PbO is red pure, orthorhombic P PbO is yeUow and crystals of lead sulfide, PbS, have a black, metallic luster. 

The reduction of molybdate salts in acidic solutions leads to the formation of the molybdenum blues (9). Reductants include dithionite, staimous ion, hydrazine, and ascorbate. The molybdenum blues are mixed-valence compounds where the blue color presumably arises from the intervalence Mo(V) — Mo(VI) electronic transition. These can be viewed as intermediate members of the class of mixed oxy hydroxides the end members of which are Mo(VI)02 and Mo(V)0(OH)2 [27845-91-6]. MoO and Mo(VI) solutions have been used as effective detectors of reductants because formation of the blue color can be monitored spectrophotometrically. The nonprotonic oxides of average oxidation state between V and VI are the molybdenum bronzes, known for their metallic luster and used in the formulation of bronze paints (see Paint). 

The primary routes of entry for animal exposure to chromium compounds are inhalation, ingestion, and, for hexavalent compounds, skin penetration. This last route is more important in industrial exposures. Most hexavalent chromium compounds are readily absorbed, are more soluble than trivalent chromium in the pH range 5 to 7, and react with cell membranes. Although hexavalent compounds are more toxic than those of Cr(III), an overexposure to compounds of either oxidation state may lead to inflammation and irritation of the eyes, skin, and the mucous membranes associated with the respiratory and gastrointestinal tracts. Skin ulcers and perforations of nasal septa have been observed in some industrial workers after prolonged exposure to certain hexavalent chromium compounds (108—110), ie, to chromic acid mist or sodium and potassium dichromate. 

Iron in the oxidation state + III in dimeric p-oxo compounds 5 can be reduced to iron + II, leading to monomeric phthalocyanines 6.351... 

It may be mentioned that the possibility of bivalence of tin in grey tin and the mercury alloy, suggested by the bipositive oxidation state of the element in many of its compounds, is ruled out because it leads to too small a value of R 1)—smaller than that for quadrivalent tin, whereas a larger value would be expected as the result of the appropriation of much of the s orbital by the unshared pair. 

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