Effects of Complexation

The bound form of a given ligand is usually very different in properties compared to the same ligand in the free state. A knowledge of the behavior of organic 

TABLE 2.7 Effect of Electronic Character of Metal Fragment on Tendency for Attached Ligand to Undergo Nucleophilic or Electrophilic Attack 

Unrcactivc May allow nu. attack May allow both nu. and el. attack May allow et. attack 

In the complex (QHg)Cr(CO)3, in contrast, the Cr(CO)3 fragment, as a good acceptor by virtue of its three CO ligands, depletes the electron density on the aromatic ring. This makes the bound suscep- 

The general situation is summarized in Table 2.8. If a ligand is normally reactive toward, say, nucleophiles, we can deactivate it by binding to a nucleophilic metal. The metal can then act as a protecting group. A ligand that is inert toward nucleophilic attack can be activated by binding to an electrophilic metal. Protection requires a stoichiometric amount of metal to be effective, so has fallen out of favor, while activation needs only a catalytic amount. 

TABLE 2.8 How the Electronic Character of a Metal Fragment Changes Reactivity 

Cp groups and phosphines are strong donors, and so the acetyl 2.17 in Eq. 2.13 is very largely in the carbene (see Chapter 11) form 2.18. It is subject to electrophilic attack to give 2.19  

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An appreciation of the extent to which invertebrate species may be exposed to such chemicals comes from considering the effects of complex mixtures. In the North Atlantic ecosystem alone, hundreds of pollutant chemicals have been identified. These include metals, synthetic and chlorinated organics and polycyclic aromatic hydrocarbons. Over 300 aromatic hydrocarbons have been detected in some regions of the Chesapeake Bay, and high concentrations of PCBs have been... 

Complexes of and The effect of complexation on the splitting of d orbitals is much greater in the case of second- and third-than for first-row transition elements, and the associated effects already noted for Ni are even more marked for Pd and Pi as a result, their complexes are, with rare exceptions, diamagnetic and the vast majority are planar also. Not many complexes are formed with O-donor ligands but, of the few that arc, [M(H20)4] ions, and the polymeric anhydrous acetates [Pd(02CMe)2l3 and [Pt(02CMc)2]4 (Fig. 27.10), are the most... 

First activated Effect of complex catalyst (new pathway lowers E )... 

Table 10-10 Equilibrium model effect of complex formation on distribution of metals (all concentrations are given as — log(M)). pH = 8.0, T = 25°C. Ligands pS04 1.95 pHCOa 2.76 pCOs 4.86 pCl 0.25.

Among pharmacenticals, EE2 has been the snbject of particular recent attention becanse of its ability to canse endocrine disrnption in fish, as has been described in Chapter 15. Low levels of mixtnres of beta blockers, such as propranolol, metoprol, and nadolol have been detected in snrface waters, and there have been investigations of their possible effects on aqnatic invertebrates (Huggett et al. 2002). Veterinary medicines, too, have come nnder scrntiny for example, the dramatic effects of diclofenac on vnltnres, which will be discnssed shortly. Many questions remain to be answered abont the possible ecological effects of complex mixtures of pharmaceuticals and veterinary medicines. 

The effect of complex formation on the solubility of a solid can be observed in the home. Silver dinnerware eventually becomes discolored by an unsightly black tarnish of Ag2 S, formed from the reaction of the silver surface with small amounts of H2 S present in the atmosphere. Silver sulfide is highly insoluble in water. Commercial silver polishes contain ligands that form strong soluble complexes with Ag ions. If a tarnished serving pan is rubbed with a polish, the black tarnish dissolves, returning the silver to its brilliant shine. 

Scheme 2.7 gives some examples of the control of stereoselectivity by use of additional Lewis acid and related methods. Entry 1 shows the effect of the use of excess TiCl4. Entry 2 demonstrates the ability of (C2H5)2A1C1 to shift the boron enolate toward formation of the 2,3-anti diastereomer. Entries 3 and 4 compare the use of one versus two equivalents of TiCl4 with an oxazoldine-2-thione auxiliary. There is a nearly complete shift of facial selectivity. Entry 5 shows a subsequent application of this methodology. Entries 6 and 7 show the effect of complexation of the aldehyde... 

Guilbault GG, Scheide EP. 1970. Chemisorption reactions of diisopropyl methyl phosphonate with various metal salts and the effect of complex-ion formation on the phosphorus-oxygen stretching frequency. Journal of Inorganic and Nuclear Chemistry 32(9) 2959-2962. 

Reed, B. and Nonavinakere, S., Metal adsorption by activated carbon—effect of complexing ligands, competing adsorbates, ionic strength, and background electrolyte, Sep Sci Technol, 27 (14), 1985-2000, 1992. 

McLaughlin M.J., Andrew S.J., Smart M.K., Smolders E. Effects of sulfate on cadmium uptake by Swiss chard I. Effects of complexation and calcium competition in nutrient solutions. Plant Soil 1998a 202 211-216. 

Belyaev, Yu. A. (1969). "i41Am distribution in rats and the effect of complexing substances on its elimination, page 168 in Radioactive Isotopes and the Body, Report No. AEC-tr-7195, Moskalev, Yu.I., Ed. (National Technical Information Service, Springfield, Virginia). 

Strong suppression of matrix effect of complex biological solution on the detection of the one component of interest... 

Coordinate bonds between metals and ligands result in the formation of complexes under many different types of conditions. In some cases, complexes form in the gas phase, and the number of known solid complexes is enormous. However, it is in solutions that many of the effects of complex formation are so important. For example, in qualitative analysis, AgCl precipitates when a solution of HC1 is added to one containing Ag+. When aqueous ammonia is added, the precipitate dissolves as a result of the formation of a complex,... 

These studies were initiated by Higuchi, who studied the effect of complexation on the solubility of numerous drug compounds. The results obtained for more than 500 systems were cited in the primary review of this work [50]. 

The effectiveness of complexes metal-acetylacetonate with tris(l,l-dimethylethyl-4-methylphenyl) phosphite in their reaction with peroxyl radicals of styrene and tetralin (323 K) decreases in the row Co2+ > V02+ > Cr3+ > Fe2+ [88]. 

It is interesting to compare the effects of complexing in the three waters we have studied so far. As shown in Table 6.10, the complexed fraction of each of the... 

Table 6.3 The effect of complexation on half-wave potentials (volts)...

The behaviour of natural ligands has been discussed in Section 4.3.3. In addition to the direct effect of complexation that is related to a decrease in the free ion activity, it has been shown that some ligands, in particular the HS, can be sorbed directly to biological surfaces, in the presence or absence of the trace metal [228,229]. This result is likely due to the fact that HS and similar macromolecules contain hydrophobic moieties that facilitate their adsorption to the plasma membrane and cell wall [157,230,231]. Because adsorption is expected to occur primarily with sites that are independent of the transporters,... 

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