The metals and carbon

The last two pages dealt with ionic and molecular solids. Now we come to some other types of solid the metals and carbon. 

In a metal, the atoms are packed tightly together in a regular pattern. The tight packing causes outer electrons to get separated hrom their atoms, and the result is a lattice of ions in a sea of electrons. Copper is a good example  

The copper ions are held together by their attraction to the electrons between them. (Opposite charges attract.) 

The regular arrangement of ions results in a crystal of copper. This picture shows copper magnified 100 times. The dark lines are the crystal edges. 

A piece of copper wire contains mMons of tiny crystals joined together. 

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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). 

Easily decomposed, volatile metal carbonyls have been used in metal deposition reactions where heating forms the metal and carbon monoxide. Other products such as metal carbides and carbon may also form, depending on the conditions. The commercially important Mond process depends on the thermal decomposition of Ni(CO)4 to form high purity nickel. In a typical vapor deposition process, a purified inert carrier gas is passed over a metal carbonyl containing the metal to be deposited. The carbonyl is volatilized, with or without heat, and carried over a heated substrate. The carbonyl is decomposed and the metal deposited on the substrate. A number of papers have appeared concerning vapor deposition techniques and uses (170—179). 

In these reactions there is a net generation of the gases CO and C02 this leads to a bulk flow of the gas mixture from the reaction zone. There are other types of reactions involving gaseous intermediates, in which there is no net gas generation. An example is the formation of a metal carbide from the metal and carbon ... 

The acid reflux procedure was first described by Rinzler el al. [28], in which raw nanotube materials are refluxed in nitric acid to oxidize the metals and carbon impurities. Acid-treated CNTs are considered to have carboxylic acid groups at the tube ends and, possibly, at defects on the side walls. The functionalized SWNTs have considerably different properties from those of the pristine tubes. 

A prerequisite for the a-elimination is the absence of (3-hydrogen atoms in the alkyl groups and this was successfully achieved by using the neopentyl substituents at the metal centre. The nature of the double bond between the metal and carbon was established by its bond length and the occurrence of stereoisomers [13], Typical feature of the Schrock carbenes is that they contain an electrophilic, high-valent metal atom and an electron rich carbene carbon atom. The reverse is true for the older, Fischer carbene compounds, such as the one mentioned, (OC)5W=CPh2. 

We now proceed to describe some selected reactions that can be understood within this framework of cr-, ir-, and carbene-type bonding between the metal and carbon. 

The Demex process is a solvent extraction demetallizing process that separates high metal vacuum residuum into demetallized oil of relatively low metal content and asphaltene of high metal content (Table 8-5) (Houde, 1997). The asphaltene and condensed aromatic contents of the demetallized oil are very low. The demetallized oil is a desirable feedstock for fixed-bed hydrodesulfurization and, in cases where the metals and carbon residues are sufficiently low, is a desirable feedstock for fluid catalytic cracking and hydrocracking units. 

Other studies have shown that rapid exchange occurs at room temperature between an alkyllithium and a lithium halide in ether or tetrahydrofuran solutions (88, 146). This exchange can be stopped at low temperature with the formation of mixed alkyllithium-lithium halide complexes. Further studies have shown that when these systems are enriched with 13C, 7Li-13C coupling can be observed at low temperatures (38). While this clearly shows the interaction which occurs between the metal and carbon atoms,... 

The pi bond between the metal and carbon atom arises as a result of a backbonding interaction between a filled metal rf-orbital of the appropriate symmetry and the empty pi star orbital of the carbonyl. Any factor that increases the electron density at the metal will increase the strength of the M C pi bond, and thus the contribution from resonance form b. The result is a... 

Because of the increased sulfur and impurity levels in crudes currently being processed, refiners in recent years have been considering residue desulfurization units upstream of the delayed coker. In addition to the reduction in sulfur content, residue desulfurization units also lower the metals and carbon residue contents. Due to the reduction in the carbon residue, the liquid product yield is increased and the coke yield reduced. In addition, the coke produced from a desulfurized residue may be suitable for use as anode grade coke. Table I shows the yields and product properties after coking Medium Arabian vacuum residue, with and without upstream residue desulfurization. 

Several structures may be envisioned for compounds MLx 2( r-C ), which are broadly distinguished by whether or not they describe closed-shell singlet states or open-shell triplet states, and the number and combination of bonds between the metal and carbon as well as adjacent carbon centers. There have been several reports rationalising the valence bond descriptions of complexes containing even387 389 or odd values of / 390,391 and related polymeric materials and Sponsler has offered a comparison of the... 

The a-bonded M-Cra-M framework is similar in all complexes, and the variation in valence bond descriptions of the M-C and C-C bonds, and associated bond orders, are associated with the changes in the composition and occupancy of the ji- and 8-frontier orbitals. In general terms, it is therefore possible to forecast the valence structures for the Cra-bridged complexes (n even) based on the metal dx configuration, since a total of [2(x—l) + 2n] electrons is contributed to the n-skeleton from the metal and carbon fragments. 

Unsaturated hydrocarbons were, however, known to be able to form complexes with the halides of some of the platinum metals and with those of copper, silver, and mercury. In such compounds the bonding of the organic residue was sometimes taken to be purely coordinative, as in (PtCla 02114)2, and sometimes a-bonding between the metal and carbon, as in CuCeHs or Pt(0113)4 was assumed. 

When the metal is niobium, the two metal frontier orbitals, dy and dz2, are closer in energy and match the corresponding orbitals of triplet free carbene. Figure 10-4b provides a picture of the interactions involved (only the important orbital interactions are shown). The elections in the M-C bond are much more equally distributed between the metal and Ccarbene (because the energy levels of the metal and carbon orbitals are comparable) than with an electrophilic carbene complex. In resonance terminology, this means that structure 10 becomes an important contributor to the overall structure of this particular Schrock carbene complex.11... 

The reaction conditions for the samples as well as the metal and carbon contents are presented in Table 1. 

All carbonyls are highly toxic by all routes of exposure the volatile liquid compounds present additional inhalation risk. Toxicity may be attributed to their decomposition products, the metals and carbon monoxide, essentially the toxic metabolites of all metal carbonyls. Therefore, the unstable carbonyls, especially the mononuclear complexes, which are readily susceptible to breakdown, should be treated as dangerous poisons. The polynuclear carbonyls of heavy metals may bioactivate and exhibit severe delayed effects. Oral intake of such substances can be fatal. The health hazards associated with individual compounds are discussed in the following sections. Toxicity data for most compounds of this class are not available. 

The oxides of iron consume charcoal in the reduction to the metal. A simple assumption of stoichiometry to the metal and carbon monoxide gives the following ratios which may be of use. 

As will be discussed later, the polymerization of propylene is catalyzed by either Ziegler-Natta catalyst or metallocene catalyst. In the Ziegler-Natta catalyzed polymerization, the propylene monomer accesses the reaction site between the metal and carbon atoms in two ways the head-to-tail and tail-to-head insertion. The head-to-tail insertion occurs primarily in isotactic polymerization [8-10], while the tail-to-head insertion occurs in the low-temperature catalysis with VCl4-Et2AlCl catalyst [11]. There are mostly head-to-tail insertions and very few head-to-head or tail-to-tail inversions in the main chain of polypropylene [12,13]. 

The local symmetry of the M —C bond is In this symmetry group, orbitals of the metal and carbon atoms always form a and n bonds (Table 4.1). 

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