Coordination chain reactions

Solid evidence for this type of chain mechanism (differing only in the chain propagation steps) had been earlier obtained by Margerum and his co-workers jjj study of coordination chain reactions between two metal complexes each containing one multidentate ligand, edta, trien and so on, e.g.. 

On the other hand, it was shown more recently that radicals could be generated from suitable substrates 33 in the coordination sphere of a SET active metal complex. The metal changes its oxidation state in this process by one (Fig. lib). In such a coordinative chain reaction, the coordinated radical 33A is subject to the subsequent process. The product 34 is in this case released from radical 33B by a homolytic substitution reaction (Sni) with respect to the coordinated atom or group and by SET at the metal. 

Margerum and Steinhaus showed that, with a coordination chain-reaction system, metals that form complexes with EDTA could be measured at concentration levels of 10" M with limits of about 10" M. The method is based on the exchange... 

Both of these pathways are disjunctive with respect to the complex ML and may be either adjunctive or disjunctive with respect to the reaction of the intermediate free M or L with the complex M L. The complete dissociation of both initial complexes is an unlikely pathway unless both are weak as may be the case for alkaline earth metal complexes. Double-exchange reactions of transition metal complexes often involve coordination chain mechanisms (for example, further reaction of L, produced by a ligand-initiated pathway, with the initial complex ML) and the overall rates may be strongly dependent on trace concentrations of reactants that promote or terminate coordination chain reactions (Margerum et a ., 1978). 

Polyethylene (Section 6 21) A polymer of ethylene Polymer (Section 6 21) Large molecule formed by the repeti tive combination of many smaller molecules (monomers) Polymerase chain reaction (Section 28 16) A laboratory method for making multiple copies of DNA Polymerization (Section 6 21) Process by which a polymer is prepared The principal processes include free radical cationic coordination and condensation polymerization Polypeptide (Section 27 1) A polymer made up of many (more than eight to ten) amino acid residues Polypropylene (Section 6 21) A polymer of propene Polysaccharide (Sections 25 1 and 25 15) A carbohydrate that yields many monosacchande units on hydrolysis Potential energy (Section 2 18) The energy a system has ex elusive of Its kinetic energy... 

Chain-reaction mechanisms differ according to the nature of the reactive intermediate in the propagation steps, such as free radicals, ions, or coordination compounds. These give rise to radical-addition polymerization, ionic-addition (cationic or anionic) polymerization, etc. In Example 7-4 below, we use a simple model for radical-addition polymerization. 

The overall objective of these studies is to unravel mechanisms of interfacial PT. This requires identification of collective coordinates (or reaction coordinates) and transition pathways of transferring protons. Differences in activation energies and rates of corresponding mechanism due to distinct polymer constituents, acid head groups, side chain lengths, side chain densities, and levels of hydration have to be examined. Comparison with experimental... 

Ionic Chain-Reaction and Complex Coordination Polymerization (Addition Polymerization) 135... 

A kinetic chain reaction usually consists of at least three steps (1) initiation, (2) propagation, and (3) termination. The initiator may be an anion, a cation, a free radical, or a coordination catalyst. Although coordination catalysts are the most important commercially, the ionic initiators will be discussed first in an attempt to simplify the discussion of chain-reaction polymerization. 

The substitutionally labile complex may be generated not only by reduction but by oxidation as well. An immediate relationship of such a reaction to the ac electrolysis proceeding without generation of excited states can be recognized. The initial production of the substitutionally labile oxidation state of ML can be achieved electrochemically (67-76), chemically (75-77) or photochemically (78). In the electrochemical experiments reduction or oxidation was accomplished by a direct current. In most cases these processes are catalytic chain reactions with Faradaic efficiencies much larger than unity. Electrochemical substitution of M(CO), with M = Cr, Mo, W was carried out by cathodic reduction to M(CO) which dissociates immediately to yield M(CO). Upon anodic reoxidation at the other electrode coordinatively unsaturated M(CO), is formed and stabilized by addition of a ligand L to give M(CO)5L (68). 

Fig. 11 Transition metal catalyzed in radical reactions based on the involved electron transfer steps, (a) Redox chain reactions by metal-catalyzed single electron transfer, (b) Coordinative radical reactions by metal-catalyzed single electron transfer...

The templates can be simply coordinated rather than attached. For example, complex 100 directed the radical relay chlorination to C-9, although the process was not as clean as with the attached templates [173]. We also used template-directed chlorina-tions to determine the conformations of flexible chains, just as we had previously with the benzophenone probes [174]. Also, by use of a set of tandem free radical chain reactions we could direct the formation of carbon-bromine and carbon-sulfur bonds, again with geometric control by the attached template [175]. 

There are a number of Se2 reactions which are not open-chain reactions. The electrophile is typically an aldehyde coordinated at the time of reaction to an electropositive atom like boron, tin or zinc on the stereogenic centre. These reactions usually use cyclic, chair-like transition structures, are called metallo-ene reactions, and are inherently syn overall. 

The mechanism of this reaction resembles that of many coordinated ligand reactions in that the positive charge on the metal draws electrons 11, 16) from the phosphate and further labilizes the weakest link in the chain, which happens in this instance to be the 5 -linkage. 

In this section of the review of crystallization during polymerization examples of chain reactions following radical, ionic and coordination mechanisms, and reactions by heterogeneous catalysis are treated. 

Double exchange reactions of transition metals often involve coordination chain mechanisms (e.g., further reaction of Y, produced by a ligand-initiated pathway, with the initial complex CaX) (Margerum et al., 1978). 

The discovery of nuclear fission in 1938 proved the next driver in the development of coordination chemistry. Uranium-235 and plutonium-239 both undergo fission with slow neutrons, and can support neutron chain reactions, making them suitable for weaponization in the context of the Manhattan project. This rapidly drove the development of large-scale separation chemistry, as methods were developed to separate and purify these elements. While the first recovery processes employed precipitation methods (e.g., the bismuth phosphate cycle for plutonium isolation). 

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