Complexes of lead

Roe, A.L. Hayes, K.F. Chisholm-Brause, C.J. Brown, G.E.Jr. Parks, G.A. (1991) In situX-ray absorption study of lead complexes at -FeOOH/water interfaces. Langmuir 7 367-373... 

Scally, S., H. Zhang, and W. Davison. 2004. Measurements of lead complexation with organic ligands using DGT. Aust. J. Chem. 57 925-930. 

Roe AL, Hayes KF, Chisholm-Brause CJ, Brown GE Jr, Hodgson , Parks GA, Leckie JO (1991) X-ray absorption study of lead complexes at a-FeOOH/water interfaces. Langmuir 7 367-373 Roedder E (1984) Fluid Inclusions. Rev Mineral Vol. 12, Mineralogical Society of America, Washington D.C. 

The kinetics of the reactions of lead complexes (Section IV) in aqueous solutions and mixed media is of equal importance to understanding the fundamental environmental (Section V) and biological chemistry (Section VI) of lead. It is important to note that although lead forms very thermodynamically stable complexes, ligands bound to lead tend to be extremely labile in aqueous solutions The water-exchange rate for lead is 7 x 10 s (49). 

Given these correlations, it seems that it should be possible to predictably turn on or off the stereochemical activity of the lead lone pair. To test this idea, Reger and co-workers prepared a series of lead complexes of polypyr-azolyl borate and polypyrazolyl methane ligands in which the number of donor atoms and the size of the ligand was systematically varied (177, 180, 183, 208). For a constant coordination number (6), only the bulkier hgand is holodirected (Fig. 12). This observation is consistent with the idea that the natural (i.e., energetically favored) state for Pb(ll) compounds is one in which the 6s and 6p orbitals are hybridized and the lone pair is stereochemically active, but that sufficient steric repulsion can overcome this effect and make the holodirected structure more stable. 

To test whether it is possible to design an improved chelating agent for lead starting from the EDTA framework, we prepared a series of lead complexes of amido derivatives of EDTA, with the goal of softening the donor set available to lead (39). Amido functional groups were selected because of earher reports... 

From studies on the structures of lead complexed to small biomolecules, we hope to gain insights into the possible coordination chemistry and speciation of lead in a biological milieu. Unfortunately, only a few structures of Pb(ll) with small biomolecules have been deposited in the CSD to date. The structures that... 

Small molecules (e.g., buffer) and pH can have a pronounced effect on lead-ligand exchange rates and are expected to have a profound effect on the kinetics of lead complexes in biological and environmental systems. 

The result common to aU the studies discussed above, both the historical sediment records and the studies examined by Thomas et al. (20), is the dramatic decrease in lead deposition in the past 15 years (7, 8, 15, 33, 417, 419 30). Whether the drop is attributable mainly to decreased gasoline emissions or to a combination of decreases in automobile, industrial, and other emissions, the existence of the trend is unambiguous. The large quantities of anthropogenic lead already in the environment, though, require continued attention. The remediation of contaminated soil and water, the treatment of lead poisoning, and the elucidation of the mechanisms of lead toxicity are all areas in which a firm understanding of the fundamental chemistry of lead complexes can contribute dramatically. 

Pyrolysis curves of lead complexes with 24) Ihlonallde. 

The lead can be recovered from the dithizone-chloroform solution by extraction with 1 per cent nitric acid or the excess of dithizone in the extract may be removed by shaking with potassium cyanide solution and the red solution of lead complex finally compared colorimetrically with a standard. 

NOTE, the use of lead oxide or salts is NOT RECOMMENDED. Information provided on the use of lead in CSM compounds in this chapter is ONLY to serve as a basis for conversion to other acid acceptors and cross-linking agents. OSHA and EPA regulations dictate the discontinued use of lead complexes in CSM and ACSM compounds. 

The importance of steric factors in the formation of penetration complexes is made evident by the observation that although sodium cetyl sulfate plus cetyl alcohol gives an excellent emulsion, the use of oleyl alcohol instead of cetyl alcohol leads to very poor emulsions. As illustrated in Fig. XIV-3, the explanation may lie in the difficulty in accommodating the kinked oleyl alcohol chain in the film. 

Hence, if chlorine is passed into a cold suspension (in hydrochloric acid) of lead(II) chloride, lead(IV) chloride is formed. Addition of ammonium chloride gives the complex salt ammonium hexachloro-plumbate(lV) as a yellow precipitate ... 

Thallium occurs in crooksite, lorandite, and hutchinsonite. It is also present in pyrites and is recovered from the roasting of this ore in connection with the production of sulfuric acid. It is also obtained from the smelting of lead and zinc ores. Extraction is somewhat complex and depends on the source of the thallium. Manganese nodules, found on the ocean floor, contain thallium. 

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

The carbopalladation is extended to homoallylic amines and sulfides[466. Treatment of 4-dimethylamino-l-butene (518) with diethyl malonate and Li2PdCl4 in THF at room temperature leads to the oily carbopalladated complex 519, hydrogenation of which affords diethyl 4-(dimethylamino) butylmalonate (520) in an overall yield of 91%. Similarly, isopropyl 3-butenyl sulfide (521) is carbopalladated with methyl cyclopentanonecarboxylate and Li2PdCl4. Reduction of the complex affords the alkylated keto ester 522 in 96% yield. Thus functionalization of alkenes is possible by this method. 

Direct Titrations. The most convenient and simplest manner is the measured addition of a standard chelon solution to the sample solution (brought to the proper conditions of pH, buffer, etc.) until the metal ion is stoichiometrically chelated. Auxiliary complexing agents such as citrate, tartrate, or triethanolamine are added, if necessary, to prevent the precipitation of metal hydroxides or basic salts at the optimum pH for titration. Eor example, tartrate is added in the direct titration of lead. If a pH range of 9 to 10 is suitable, a buffer of ammonia and ammonium chloride is often added in relatively concentrated form, both to adjust the pH and to supply ammonia as an auxiliary complexing agent for those metal ions which form ammine complexes. A few metals, notably iron(III), bismuth, and thorium, are titrated in acid solution. 

PbO PbO H2O, for example, may be formed by boiling suspensions of lead oxide and lead sulfate in water. In addition, complex mixed salts, such as white lead, 2PbC02 Pb(OH)2, are readily formed. 

Lead is toxic to the kidney, cardiovascular system, developiag red blood cells, and the nervous system. The toxicity of lead to the kidney is manifested by chronic nephropathy and appears to result from long-term, relatively high dose exposure to lead. It appears that the toxicity of lead to the kidney results from effects on the cells lining the proximal tubules. Lead inhibits the metaboHc activation of vitamin D in these cells, and induces the formation of dense lead—protein complexes, causing a progressive destmction of the proximal tubules (13). Lead has been impHcated in causing hypertension as a result of a direct action on vascular smooth muscle as well as the toxic effects on the kidneys (12,13). 

However, when either P(CgH )(CH2)2 or P(CgH )2(CH2) is used to form cis- or /n j -M(N2)2(PR3)4j M = Mo or W, respectively, followed by treatment with acid, ammonia yields of about 2 mol or 0.7 mol pet mole of complex for M = W and Mo, respectively, are produced (193,194). These and related data have been used to suggest a possible stepwise sequence for the reduction and protonation of N2 on a single molybdenum atom ia nitrogeaase (194). However, acidificatioa leads to complete destmctioa of the complex. Using both the stabilizing effect of the chelating phosphine triphos,... 

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