Complex transient

We have no direct evidence that the transient complexes (Ci8H39+) and (C20H43+) actually exist, and they are used primarily for convenience in writing the equations. However, transient association complexes do occur in low energy ion-molecule reactions, and their actual existence in this case might be expected. 

The existence of a transient complex between malic acid (HA) and Co(IlI) has been demonstrated optically 275 nm) . 

Stable heterologous complexes are not necessary to explain the limited P-enolpyr-uvate-dependent mannitol phosphorylation kinetic data now available from domain complementation assays transient complexes between domains are sufficient. The challenge remains, however, to visualize how a subcloned A or B domain would be able to transiently associate with an A or B domain on an dimer or tetramer,... 

Some time ago experimental evidence for the electrochemical generation of a transient complexed Hg111 species was given,74 but never since has an Hg111 compound been isolated and studied. Also, compounds with HgIV, corresponding to the well-known Au111, have not yet been prepared. 

The ability of complexes to catalyze several important types of reactions is of great importance, both economically and intellectually. For example, isomerization, hydrogenation, polymerization, and oxidation of olefins all can be carried out using coordination compounds as catalysts. Moreover, some of the reactions can be carried out at ambient temperature in aqueous solutions, as opposed to more severe conditions when the reactions are carried out in the gas phase. In many cases, the transient complex species during a catalytic process cannot be isolated and studied separately from the system in which they participate. Because of this, some of the details of the processes may not be known with certainty. 

The kinetically-stabilized complexes of the cage ligands normally yield redox reagents free of the exchange problems often associated with simple complexes. Indeed, the redox chemistry of the complexes shows a number of unusual features for example, saturated cages of the type mentioned in Chapter 3 are able to stabilize rare (monomeric) octahedral Rh(n) species (d7 electronic configuration) (Harrowfield etal., 1983). In a further study, radiolytical or electrochemical reduction of the Pt(iv) complexes of particular cages has been demonstrated to yield transient complexes of platinum in the unusual 3+ oxidation state (Boucher et al., 1983). 

Complexes of enzyme, substrates, products, inhibitors, etc., are often designated as being binary, ternary, quaternary, etc., depending on the number of entities present in the complex. For example, EAB would be a ternary complex. Central complexes are those transient complexes that generate products (or substrates in the reverse reaction) or which isomerize to those forms which can generate products. Thus, in an ordered Bi Bi reaction scheme, the enzyme can exist in five forms E, EA, EAB,... 

It was Henri who first proposed that enzyme catalysis depended on the formation of a transient complex of enzyme and substrate, followed by the breakdown i.e., chemical conversion) of bound substrate into product. Nonetheless, credit for derivation of the rate expression for the initial rate phase of one-substrate enzyme-catalyzed reactions is given to Michaelis and Menten. Both treatments gave the same general result ... 

With this system, we finally succeeded in characterizing the first rhodium dihydride species in the asymmetric hydrogenation of enamides. Additionally, we succeeded afterwards in the characterization of all the possible catalyst dihydride species [39]. In subsequent work, now knowing what to look for and where to look, all transient complexes in the asymmetric enamide hydrogenation with the Rh(PHA-NEPHOS) catalyst could also be observed with classical NMR techniques [37]. 

Betterton, E. A., On the pH-Dependent Formation Constants of Iron(III) SulfurflV) Transient Complexes, . /. Atmos. Chem., 17, 307-324 (1993). 

B. Acid Dissociation Constants for Transient a-Hydroxyalkyl Complexes Mechanisms of Decomposition of the Transient Complexes LmM +1-R... 

Bimolecular Decomposition of the Transient Complexes Methyl Transfer Reactions Rearrangement of the Carbon-Skeleton of R (H20)sCrCH02+ as a Hydride Transfer Reducing Agent Alkene Complexes... 

The nature of the central metal ion. In principle the redox potential of the Mre+1/re couple and the stabilization of the M" + 1 ion by the ligand R affect the metal-carbon bond strength. The stability of the transient complex is also affected by the rate of ligand exchange of the LmM"+1 R complex. 

The equatorial ligands, which influence the redox properties of the central metal ion, depending on their a-donor character. Thus structure and identity of the equatorial ligands affect the properties of the transient complex, i.e., the stability of the M-C bond. 

Figure 1 shows a typical time trace of formation of a transient complex as observed in the linear accelerator setup. 

Depending on the pH of the solution spectra composite of the relative amounts of acid and base form of the transient complex are observed after its formation and equilibration due to Eq. (16) and K]C> can thus be derived. 

The mechanisms and kinetics of decomposition of the transient complexes LmM + 1-R in aqueous solutions depend on the nature of the central cation, of the ligands, L, of the substituents on the aliphatic residue, R, on the pH and on the presence and nature of various substrates, S, in the medium, e.g., 02. In the following section the major mechanisms observed are discussed. 

In many systems studied the results point out that Eq. (9) is an equilibrium process and that the mechanism of decomposition of the transient complex LmMn + 1-R involves radical processes. 

Table III summarizes a recent study comparing equilibrium constants of homolysis and volumes of activation and reaction for the formation and homolysis reaction of transient complexes with metal-carbon -bonds containing different metal centers (46). Previously the large volumes of activation for the homolysis reaction that were measured for Cr111 (56), Co111 (73), Ni111 (43) (15-26 cm3 mol-1) were interpreted as indication for an ShI mechanism (i.e., due to bond breakage and to the breakup of the solvent cage due to the separation of the aliphatic radicals from the Lm 2Mre center) (127). But as an I mechanism for the forward (formation)...

Volumes of Activation and Reaction for the Formation and Homolysis of Transient Complexes with Metal-Carbon [Pg.286]

The homolytic decomposition in the absence of scavengers for the radical R is often followed by the reaction of the radical "R with the transient complex with the metal-carbon bond ... 

This reaction leads in many systems to unexpected products. The detailed mechanism of this reaction was studied only in few systems (27,28). Systems which follow this pathway (Eq. (9) followed by (27)), show an inverse dependence on [M"L, ] of their second order rate constant of decomposition of the transient complex with the metal-carbon bond. This is contrary to an inverse dependence on [M Lm]2, which is observed when reaction (9) is followed by reaction (26) (radical radical reaction). 

The transient complexes, "+1 OOR thus formed are an important class of intermediates by themselves (128-131). Moreover in several systems, which decompose via homolysis, oxygen was used as a scavenger for the methyl radical (72,84) and thus aided in the elucidation of the mechanism and the determination of the associated rate constants (72,84). 

Many electron acceptors are able to oxidize transient complexes with metal-carbon -bonds. The reaction of organochromium(III) species ((H20)5Cr-R2 + and L(H20)Cr R2 +, L =1,4,8,12-tetraazacyclo-pentadecane) have been studied for the acceptors Ru(bpy) +, 2E-Cr (bpy) +, Ni([14]aneN4)3 +, and IrCl (132,133), and proceed according to the following general equation ... 

The (3-elimination mechanism of decomposition of transient complexes of the type LmMre+1 R occurs when a good leaving group, X, e.g., X= OR, NR2, NHC(0)R, and halides, is bound to the (3-carbon ... 

Comparing rates of decomposition for transient complexes containing different metal centers (Table V) it was observed that the specific rates of (3-hydroxyl elimination for Cu" CR T CR R4 complexes are considerably higher than those observed for (H20)sCrinR, (tspc)CoinR and PPFeinR (136). 

A detailed study of the CO insertion, or methyl migration, observing formation and decomposition of the transients, was performed so far only for one Cu(I) model system (93). It was reported that methyl radicals form transient complexes containing metal carbon -bonds with carbonmonoxide (n = 1, 3, 4) complexes of Cu(I). These complexes decompose yielding Cu(II) and acetaldehyde as final products via an copper acetyl intermediate formed by insertion of /migration of CH3 as described in Scheme 4. 

In principle one can expect that the formation of transient complexes of the type LmM + 1-R might result in the rearrangement of the carbon-skeleton of R in analogy to B-12 catalyzed processes (151). However, only one such reaction was observed till now (89), probably due to the fact that very simple alkyl radicals are used in most studies. This rearrangement is discussed in Section IX. 

The final products of this reaction were still not determined. These results thus suggest that the detailed mechanism of reduction of ethylene and maleate by Ni1 / + yields as transients complexes with Ni-carbon bonds. The mechanism of decomposition of the latter depends on the nature of substituents on the alkyl radical. 

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