From Metal-Carbon Complexes

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)-...

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. 

P-Hydrogen eliminations and p-aryl eliminations from alkoxo and amido complexes are also known. Such eliminations have been shown to occur by migratory de-insertion pathways, as well as alternative p-hydride abstraction mechanisms. P-Hydrogen eliminations from metal-silyl complexes are rare because the silicon-carbon double bond in the product is weak. For similar reasons, p-hydrogen eliminations from metal-thiolate complexes are rare. 

Metal-mediated DCA between azides and isocyanides typically starts from metal-azide complexes and free isocyanides. Thus, the azido complexes [RhCp (/u-N3)(N3)]2 (Cp = rj-C Me ), trani-Rh(N3)(CO)(PPh3)2, Na2[Pd(N3)4], Na2[Pd2(/u-N3)2(N3)4], and Na[Au(N3)4], reacted with aliphatic isocyanides to give a series of new metal-carbon bonded tetrazolato complexes [40]. All azide ligands in the coordination sphere undergo this cycloaddition with isocyanides except on palladium(II), where only two tetrazol-5-ato groups are formed (12, Scheme 13.11). 

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

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). 

Tellurium and cadmium Electrodeposition of Te has been reported [33] in basic chloroaluminates the element is formed from the [TeCl ] complex in one four-electron reduction step, furthermore, metallic Te can be reduced to Te species. Electrodeposition of the element on glassy carbon involves three-dimensional nucleation. A systematic study of the electrodeposition in different ionic liquids would be of interest because - as with InSb - a defined codeposition with cadmium could produce the direct semiconductor CdTe. Although this semiconductor can be deposited from aqueous solutions in a layer-by-layer process [34], variation of the temperature over a wide range would be interesting since the grain sizes and the kinetics of the reaction would be influenced. 

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... 

Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiCU (169) and the catalyst Cr -CjHsU/SiCU (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ir-allyl complexes (ir-allyl ligands being removed from the active center) (140) C14 of the...

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

The reaction of JV,iV-dimethylhydrazones (1-amino-1-azadienes) and alkenylcarbene complexes mainly produces [3C+2S] cyclopentene derivatives (see Sect. 2.6.4.5). However, a minor product in this reaction is a pyrrole derivative which can be considered as derived from a [4S+1C] cycloaddition process [75]. In this case, the reaction is initiated by the nucleophilic 1,2-addition of the nitrogen lone pair to the metal-carbon double bond followed by cyclisation and... 

The aim of this volume is to convince the reader that metal carbene complexes have made their way from organometallic curiosities to valuable - and in part unique - reagents for application in synthesis and catalysis. But it is for sure that this development over 4 decades is not the end of the story there is both a need and considerable potential for functional organometallics such as metal carbon multiple bond species which further offer exciting perspectives in selective synthesis and catalysis as well as in reactions applied to natural products and complex molecules required for chemical architectures and material science. 

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