Metal-carbon complexes

This chapter is concerned entirely with the insertion of carbon monoxide into transition metal-carbon cr-bonds. Sulfur dioxide insertion 154, 239), also common among transition metal-carbon complexes, will be treated in a complementary review, which is to appear later. Subject to the restrictions given at the beginning of Section VI, an attempt has been made at a complete literature coverage of the insertion of CO. Particular emphasis focuses on recent results, especially those of a kinetic and stereochemical nature. 

Two aspects of porphyrin electrosynthesis will be discussed in this paper. The first is the use of controlled potential electroreduction to produce metal-carbon a-bonded porphyrins of rhodium and cobalt. This electrosynthetic method is more selective than conventional chemical synthetic methods for rhodium and cobalt metal-carbon complexes and, when coupled with cyclic voltammetry, can be used to determine the various reaction pathways involved in the synthesis. The electrosynthetic method can also lead to a simultaneous or stepwise formation of different products and several examples of this will be presented. 

Although SO2 insertion is a clean reaction for many metal-carbon complexes, some metal alkyls, e.g., PhCH2Cr(H20)s (4), decompose rapidly to unidentified materials upon treatment with sulfur dioxide. At the other end of the reactivity scale, a number of metal-carbon a-bonded compounds are inert to SO2. These include, in particular, perfluoro-alkyls and -aryls such as CF3Mn(CO)s (66) and CpFe(CO)2CeF5 (13), as well as other complexes with electron-withdrawing substituents in the o-bonded carbon fragment (71). 

Figure 6. Stability constants of some hi divalent metal carbonate complexes plotted against stability constants for the corresponding oxalate complexes. Data are from literature and are for 1 = 0. The equation of the line is log Kassoc (MCOs ) = 1.11 X log Kassoc (MC OA)-...

BRU/CAS2] Bruno, J., Casas, I., Grenthe, I., Lagerman, B., Studies on metal carbonate complexes 19. Complex formation in the Th(IV)-H20-C02(g) system, Inorg. Chim. Acta, 140, (1987), 299-301. Cited on pages 371, 594. 

The interplanar separation of Rh(OEP)In(OEP) is 3.41 A compared to 3.26 A for [Ru(OEP)]2 As is also observed for the ruthenium dimer the In(OEP) core is twisted 21.8° relative to the Rh(OEP) group. Both the indium and rhodium atoms have an out-of-plane distance A4N which is the same as that previously reported for the corresponding o-bonded metal carbon complexes (given in Table 15 of part B V). A covalent radius of 1.36 A may be estimated from the metal-carbon bond length in In(TPP)(CH3). Similarly the covalent radius of the rhodium atom in Rh(OEP)(CH3) is equal to 1.26 A, giving a predicted covalent Rh-In distance of 2.62 A. This latter value is in good agreement with the observed value (2.584(2) A). In addition, the reactivity of the dimer... 

It is the intention of the authors to present a brief account on metal carbonate complexes which have a direct bearing on the reversible hydration of CO2 by the enzyme carbonic anhydrase. Emphasis is placed on the integration of the kinetic and mechanistic concepts derived from the studies on model systems with the available kinetic, chemical and structural information on the enzyme carbonic anhydrase. To start, the kinetics and equilibria of dissolved CO2, relevant to the present context, are presented. 

Hence PM3 and ab initio methods can be used in metal-carbon complexes formations investigations. 

Electrical measurements performed by Nguyen et al. [187] indicate that the aluminum/PPV interface shows a rectifying behavior when aluminum is deposited as a top electrode, as well as a bottom electrode, after polymer conversion and before polymer conversion, respectively. XPS analyses indicate chemical reactions between the polymer and the metal in the presence of ojqrgen to form metal-carbon complexes [187]. 

Organometallic compounds which have main group metal-metal bonds, such as S—B, Si—Mg,- Si—Al, Si—Zn, Si—Sn, Si—Si, Sn—Al, and Sn—Sn bonds, undergo 1,2-dimetallation of alkynes. Pd complexes are good catalysts for the addition of these compounds to alkynes. The 1,2-dimetallation products still have reactive metal-carbon bonds and are used for further transformations. 

In a back titration, a slight excess of the metal salt solution must sometimes be added to yield the color of the metal-indicator complex. Where metal ions are easily hydrolyzed, the complexing agent is best added at a suitable, low pH and only when the metal is fully complexed is the pH adjusted upward to the value required for the back titration. In back titrations, solutions of the following metal ions are commonly employed Cu(II), Mg, Mn(II), Pb(II), Th(IV), and Zn. These solutions are usually prepared in the approximate strength desired from their nitrate salts (or the solution of the metal or its oxide or carbonate in nitric acid), and a minimum amount of acid is added to repress hydrolysis of the metal ion. The solutions are then standardized against an EDTA solution (or other chelon solution) of known strength. 

Inorganic heavy metals are usually removed from aqueous waste streams by chemical precipitation in various forms (carbonates, hydroxides, sulfide) at different pH values. The solubiUty curves for various metal hydroxides, when they are present alone, are shown in Figure 7. The presence of other metals and complexing agents (ammonia, citric acid, EDTA, etc) strongly affects these solubiUty curves and requires careful evaluation to determine the residual concentration values after treatment (see Table 9) (38,39). 

The bonding between carbon monoxide and transition-metal atoms is particularly important because transition metals, whether deposited on soHd supports or present as discrete complexes, are required as catalysts for the reaction between carbon monoxide and most organic molecules. A metal—carbon ( -bond forms by overlapping of metal orbitals with orbitals on carbon. Multiple-bond character between the metal and carbon occurs through formation of a metal-to-CO TT-bond by overlap of metal-i -TT orbitals with empty antibonding orbitals of carbon monoxide (Fig. 1). 

The nature of the bonding, particularly in CO, has excited much attention because of the unusual coordination number (1) and oxidation state (-f2) of carbon it is discussed on p. 926 in connection with the formation of metal-carbonyl complexes. 

The l ,J -DBFOX/Ph-transition metal aqua complex catalysts should be suitable for the further applications to conjugate addition reactions of carbon nucleophiles [90-92]. What we challenged is the double activation method as a new methodology of catalyzed asymmetric reactions. Therein donor and acceptor molecules are both activated by achiral Lewis amines and chiral Lewis acids, respectively the chiral Lewis acid catalysts used in this reaction are J ,J -DBFOX/Ph-transition metal aqua complexes. 

Several model systems related to metalloenzymes such as carboxypeptidase and carbonic anhydrase have been reviewed. Breslow contributed a great deal to this field. He showed how to design precise geometries of bis- or trisimidazole derivatives as in natural enzymes. He was able to synthesize a modified cyclodextrin having both a catalytic metal ion moiety and a substrate binding cavity (26). Murakami prepared a novel macrocyclic bisimidazole compound which has also a substrate binding cavity and imidazole ligands for metal ion complexation. Yet the catalytic activities of these model systems are by no means enzymic. 

When complexes of bulky tertiary phosphines are heated, internal metal-carbon bond formation frequently occurs (Figure 3.50). 

It can be concluded from the study of Grubbs and Brunck that indeed a metal-carbon cr-complex might be the key intermediate in the metathesis reaction. For the conversion of I into II several reaction pathways can be... 

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