Carbonate metal complexes

This is also especially true for the binding of carbonate and hydrogen carbonate, which can both act as monodentate and bidentate ligands. For quite some time it was not possible to isolate stable hydrogen carbonate metal complexes since these tend to undergo rapid loss of CO2. However, more recently such complexes have been isolated and well characterized. It has also been shown that, depending on the nature of the spectator ligands. 

The nature of the bonding in silylene-metal complexes, as compared with the better known metal-carbene complexes, is a question of considerable interest. MO calculations on H2Si=Mo(CO)5 indicate that the Si—Mo bond consists of a cr-donor and --backbond component, like the carbon-metal complexes. The -component is, however, weaker than for metal carbenes251. Infrared C=0 frequencies for the base-free silylene metal complexes support this model. Theoretical considerations of the bonding in silylene-metal complexes are treated more fully in Section IV.E. 

C-H activation with transition-metal complexes will open a new chemistry of catalytic carbon-carbon bond formation because of its potent ability to generate reactive carbon-metal complexes. The design of a catalytic reaction which involves C-H activation followed by reaction with a reagent such as an electrophile is particularly important to provide an environmentally friendly non-salt process, which proceeds under neutral conditions. 

Especially for alkyl halides 6 the transfer of a single electron from the metal center is facile and occurs at the halide via transition state 6C, which stabilizes either by direct abstraction of the halide to a carbon-metal complex radical pair 6D or via a distinct radical anion-metal complex pair 6E. This process was noted early but not exploited until recently (review [45]). Alkyl tosylates or triflates are not easily reduced by SET, and thus Sn2 and/or oxidative addition pathways are common. The generation of cr-radicals from aryl and vinyl halides has been observed, but is rarer due to the energy requirement for their generation. Normally, two-electron oxidative addition prevails. 

In an improved process for the synthesis of tropilidene (5) by E. Muller, a solution of diazomethane in benzene is added gradually to refluxing benzene containing cuprous bromide as catalyst. Benzene is used in large excess, and the product is isolated most easily by filtering the solution from the catalyst and adding it to a solution of phosphorus pentachloride in carbon tetrachloride. The tropylium chloride which separates is dissolved in water and treated with perchloric acid to afford tropylium perchlorate in 85% yield. The success of the method is attributed to formation of the intermediate (3), a deactivated electrophilic carbon metal complex. Tropilidene... 

It had been mentioned by Geus (8) that deposition-precipitation by evaporation of solvent is another way to gradually increase the concentration of a metal precursor in solution, but he also reported that this leads to an inhomogeneous distribution of the active material. However, ammonia evaporation of solutions containing ammine or ammine carbonate metal complexes has been reported as a way to gradually decrease the pH of the solution to induce precipitation of hydroxy carbonate compound on alumina support and to prepare Co/AljOj (43, 44), Ni/Al203 (45, 46), CU/AI2O3 catalysts (47). The preparation method was very broadly described in the patent references. It consists of a suspension of alumina... 

Many transition metal complexes including Ni(CO)4 obey the 18 electron rule, which IS to transition metal complexes as the octet rule is to mam group elements like carbon and oxygen It states that... 

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... 

Oxo Synthesis. Ad of the synthesis gas reactions discussed to this point are heterogeneous catalytic reactions. The oxo process (qv) is an example of an industriady important class of reactions cataly2ed by homogeneous metal complexes. In the oxo reaction, carbon monoxide and hydrogen add to an olefin to produce an aldehyde with one more carbon atom than the original olefin, eg, for propjiene ... 

Precipitation is affected by pH, solubiUty product of the precipitant, ionic strength and temperature of the aqueous stream, and the presence of metal complexes. For each metal precipitant, there is an optimum pH where its solubiUty is lowest and hence, the highest removals may be achieved. When an aqueous stream contains various metals, the precipitation process caimot be optimized for each metal, sometimes making it difficult to achieve effluent targets for each. SolubiUty products depend on the form of the metal compound and ate lowest for metal sulfides, reflecting the relative insolubiUty of these compounds. For example, the solubiUty product for lead sulfide [1314-87-0] is on the order of compared to 10 for lead carbonate. Metal... 

Carbon monoxide [630-08-0] (qv), CO, the most important 7T-acceptor ligand, forms a host of neutral, anionic, and cationic transition-metal complexes. There is at least one known type of carbonyl derivative for every transition metal, as well as evidence supporting the existence of the carbonyls of some lanthanides (qv) and actinides (1) (see AcTINIDES AND THANSACTINIDES COORDINATION COMPOUNDS). 

Cyclopentadiene itself has been used as a feedstock for carbon fiber manufacture (76). Cyclopentadiene is also a component of supported metallocene—alumoxane polymerization catalysts in the preparation of syndiotactic polyolefins (77), as a nickel or iron complex in the production of methanol and ethanol from synthesis gas (78), and as Group VIII metal complexes for the production of acetaldehyde from methanol and synthesis gas (79). 

According to a detailed mechanistic study, the first step is the abstraction of the relatively acidic hydrazone proton (93- 97). This is followed by hydride attack on the trigonal carbon of the C=N bond, mainly from the a-side at C-3, together with the concomitant loss of the tosylate anion (97 -> 98). Expulsion of nitrogen from the resulting intermediate (98) yields a fairly insoluble anion-metal complex (99) which upon decomposition with water provides the methylene derivative (100). 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

As well as phosphorus ligands, heterocyclic carbenes ligands 10 have proven to be interesting donor ligands for stabilization of transition metal complexes (especially palladium) in ionic liquids. The imidazolium cation is usually presumed to be a simple inert component of the solvent system. However, the proton on the carbon atom at position 2 in the imidazolium is acidic and this carbon atom can be depro-tonated by, for example, basic ligands of the metal complex, to form carbenes (Scheme 5.3-2). 

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