Complexes formation

The formation of molecular complexes in solution may lead to an apparent increase in solubility. It is important to be aware that this increase is a manifestation of the fact that the molecular complexes formed must be treated as chemical entities that are distinct from the actual solute under investigation, as having properties such as solubility different from that of the solute. 

The importance of complex formation is exemplified by the binding of Mg, Na, Co, Mn, Fe, Cu, and Zn to fulvic acids (FA), and of Fe to humic acids. The binding capacity of these natural acids for metal ions is within the range of 0.2-0.6 mmol/g, and the order of stability of complex formation (M-FA) with some key metals is Fe2+ Al3+ Cu2+ Ni2+ Ca2+ Zn2+ Mn2+ Mg2+(see Schnitzer, 1970). Interestingly, some cryptogams (i.e., mosses and lichens) capture part of their essential minerals by secreting 

Complexation of metal ions by ligands in solution can be in competition with the binding of metals onto solid surfaces. The final result will depend on 

For example, in the presence of strongly binding natural organic ligands in water (especially at pH 6), metals such as Cu, Ni, and Cd remain in solution, decreasing their extent of adsorption onto solid particles (e.g., goethite). On the contrary, with weaker ligands (even in excess), and more acidic pH, the metal ions prefer to adsorb onto soil or sediment particles. 

Another example involves complex oxidation with the participation of natural catalysts such as goethite (ct-FeOOH), a redox-reactive solid  

Phenomena such as chemical and biological transformations, metal mobility, bioavailability, bioaccumulation, toxicity, and persistence in the environment frequently depend on the chemical form or speciation of a given ion, especially the metallic ions. For example, there is normally a great difference between the sorption behavior of a free metal cation and that of its anionic complexes onto mineral oxides and hydroxides. 

An important step in the catalytic reactions of alkenes is the complexation of the substrate at the transition metal center. Differences in ability of olefins to coordinate can influence the selectivity of a catalytic process to such an extent that, for example, in a positionally isomeric olefins, the terminal olefins react preferentially to give the desired product. 

In alkene complexes, the transition metal can have oxidation state 0 or higher. The olefin ligands are bound to the transition metal through one or more double bonds, the exact number depending on the number of free sites in the electron shell of the metal atom. Generally sufficient olefins or other Lewis bases are added to 

Cyclooctatetraene irontricarbonyl. Formal Fe charge 0. Number of Jt electrons involved 4 

In olefin-metal bonding, a distinction is made between o and it bonding contribi-tions. The 7t bonding contribition for several metals increases as follows  

We now inquire into the nature of solvent effeets on ehemieal equilibria, taking noneovalent moleeular eomplex formation as an example. Suppose speeies S (substrate) and L (ligand) internet in solution to form eomplex C, Kn being the eomplex binding eonstant. 

37] gives the free energy with respect to a IM standard state, because the unit of Kn is M . To calculate the unitary (mole fraction) free energy change we write, instead of eq. [5.5.37], eq. [5.5.38]  

42] says that the solvent effect on complex formation is a function solely of the solvent effects on the solubilities of reactants (negative signs) and product (positive sign). This is a powerful result, because we already have a detailed expression, eq. [5.5.23], for each of 

In praetiee, of eourse, there are diffieulties. Each of the 2 lAGsol terms contains three adjustable parameters, for nine in all, far too many for eq. [5.5.42] to be practicable in that form. We therefore introduce simplifications in terms of some special cases. The first thing to do is to adopt a l-step model by setting Kj = 0. This leaves a six-parameter equation, which, though an approximation, will often be acceptable, especially when the experimental study does not cover a wide range in solvent composition (as is usually the case). This simplification gives eq. [5.5.43]. 

The particular example of cyclodextrin complexes led to the identification of another special case as the partial cancellation approximation-, in this case we assume = Kj Ki, and the result is, approximately.  

Journal of the Chemical Society. Perkin Transactions 2,. reference 17.) 

In practice, of coxuse, there are difficulties. Each of the terms contains three 

When a complex is formed there is an intimate chemical interaction between the ions, or between an ion and an uncharged ligand. This is in contrast to the purely electrostatic interaction which results in the formation of an ion pair. There are situations when an unambiguous assignment of a species as an ion pair or as a complex may be difficult. These matters are discussed in Sections 1.20 and 1.21. 

However, no matter which species are considered, the equilibrium calculations involved are identical. 

be aware in complex formation two formulations are typically used for the equilibrium expression, and it is crucial to be aware of the distinctions. Equilibria are either given in terms of step-wise formation equilibrium constants of successive complexes, Ks, or in terms of the overall formation constant of a given complex from the metal ion and the iigand, s. 

Direct chemical interaction between a metal ion and a ligand often results in a series of complexes. In complex formation the ligand replaces the water coordinated to the metal ion. 

In standard treatments of complex formation equilibria, instead of individual, or step-wise, Ks defined as above, overall equilibrium constants are commonly used. The symbol is used for such equilibrium constants. In the present context, the sequence of reactions are defined as  

If anion X- precipitates metal M+, it is often observed that a high concentration of X causes solid MX to redissolve. The increased solubility arises from the formation of complex ions, such as MX2, which consist of two or more simple ions bonded to each other. 

In complex ions such as Pbl+, Pblj, and Pbl, iodide is said to be the ligand of Pb2f. A ligand is any atom or group of atoms attached to the species of interest. We say that Pb2+ 

The product of the reaction between a Lewis acid and a Lewis base is called an adduct. The bond between a Lewis acid and a Lewis base is called a dative or coordinate covalent bond. 

Lewis acid Lewis base adduct Electron pair Electron pair acceptor donor 

If Pb2+ and I only reacted to form solid Pbl2, then the solubility of Pb2+ would always be very low in the presence of excess I-. 

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The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

Quack M and Troe J 1975 Complex formation in reactive and inelastic scattering statistical adiabatic channel model of unimolecular processes III Ber. Bunsenges. Phys. Chem. 79 170-83... 

Compare this reaction with (2) of the oxidising examples, where iron(II) is oxidised to iron(III) in acid solution change of pH, and complex formation by the iron, cause the complexed iron(III) to be reduced.)... 

These can be prepared by electrolytic oxidation of chlorates(V) or by neutralisation of the acid with metals. Many chlorates(VII) are very soluble in water and indeed barium and magnesium chlorates-(VII) form hydrates of such low vapour pressure that they can be used as desiccants. The chlorate(VII) ion shows the least tendency of any negative ion to behave as a ligand, i.e. to form complexes with cations, and hence solutions of chlorates (VII) are used when it is desired to avoid complex formation in solution. 

However the Mn (aq) ion can be stabilised by using acid solutions or by complex formation it can be prepared by electrolytic oxidation of manganese(II) solutions. The alum CaMn(S04)2.12H2O contains... 

It is dissolved by bromine trifluoride, to form finally gold(III) fluoride, AuFj. This is a notable compound, for in it gold exhibits a simple valency of three, whereas in all other gold(III) compounds, gold is 4-coordinate, usually by complex formation (see below). 

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

Conformational Adjustments The conformations of protein and ligand in the free state may differ from those in the complex. The conformation in the complex may be different from the most stable conformation in solution, and/or a broader range of conformations may be sampled in solution than in the complex. In the former case, the required adjustment raises the energy, in the latter it lowers the entropy in either case this effect favors the dissociated state (although exceptional instances in which the flexibility increases as a result of complex formation seem possible). With current models based on two-body potentials (but not with force fields based on polarizable atoms, currently under development), separate intra-molecular energies of protein and ligand in the complex are, in fact, definable. However, it is impossible to assign separate entropies to the two parts of the complex. 

The solubility of hydrogen chloride in solutions of aromatic hydrocarbons in toluene and in w-heptane at —78-51 °C has been measured, and equilibrium constants for Tr-complex formation evaluated. Substituent effects follow the pattern outlined above (table 6.2). In contrast to (T-complexes, these 7r-complexes are colourless and non-conducting, and do not take part in hydrogen exchange. 

Formation of a Tr-allylpalladium complex 29 takes place by the oxidative addition of allylic compounds, typically allylic esters, to Pd(0). The rr-allylpal-ladium complex is a resonance form of ir-allylpalladium and a coordinated tt-bond. TT-Allylpalladium complex formation involves inversion of stereochemistry, and the attack of the soft carbon nucleophile on the 7r-allylpalladium complex is also inversion, resulting in overall retention of the stereochemistry. On the other hand, the attack of hard carbon nucleophiles is retention, and hence Overall inversion takes place by the reaction of the hard carbon nucleophiles. 

Reactions Involving Pd(II) Compounds and Pd(0) Complexes ic-Allyl complex formation and its reaction with a nucleophile... 

Treatment of 7r-allylpalladium chloride with CO in EtOH affords ethyl 3-butenoate (321)[284]., 3, y-Unsaturated esters, obtained by the carbonylation of TT-allylpalladium complexes, are reactive compounds for 7r-allyl complex formation and undergo further facile transformation via 7r-allylpalladium complex formation. For example, ethyl 3-butenoate (321) is easily converted into 1-carboethoxy-TT-allylpalladium chloride (322) by the treatment with Na PdCL in ethanol. Then the repeated carbonylation of the complex 322 gives ethyl 2-... 

Chlorination of the azobenzene complex 463 with chlorine produces mono-chloroazobenzene with regeneration of PdCN. Then complex formation takes place again with the chlorinated azobenzene. By this sequence, finally tetra-chloroazobenzene (503) is obtained using a catalytic amount of PdCT. The reaction, carried out by passing chlorine gas into an aqueous dioxane solution of azobenzene and PdCf for 16 h, gives a mixture of polychlorinated azoben-zenes[455]. 

The stereochemistry of the Pd-catalyzed allylation of nucleophiles has been studied extensively[5,l8-20]. In the first step, 7r-allylpalladium complex formation by the attack of Pd(0) on an allylic part proceeds by inversion (anti attack). Then subsequent reaction of soft carbon nucleophiles, N- and 0-nucleophiles proceeds by inversion to give 1. Thus overall retention is observed. On the other hand, the reaction of hard carbon nucleophiles of organometallic compounds proceeds via transmetallation, which affords 2 by retention, and reductive elimination affords the final product 3. Thus the overall inversion is observed in this case[21,22]. 

Based on the above-mentioned stereochemistry of the allylation reactions, nucleophiles have been classified into Nu (overall retention group) and Nu (overall inversion group) by the following experiments with the cyclic exo- and ent/n-acetales 12 and 13[25], No Pd-catalyzed reaction takes place with the exo-allylic acetate 12, because attack of Pd(0) from the rear side to form Tr-allyl-palladium is sterically difficult. On the other hand, smooth 7r-allylpalladium complex formation should take place with the endo-sWyWc acetate 13. The Nu -type nucleophiles must attack the 7r-allylic ligand from the endo side 14, namely tram to the exo-oriented Pd, but this is difficult. On the other hand, the attack of the Nu -type nucleophiles is directed to the Pd. and subsequent reductive elimination affords the exo products 15. Thus the allylation reaction of 13 takes place with the Nu nucleophiles (PhZnCl, formate, indenide anion) and no reaction with Nu nucleophiles (malonate. secondary amines, LiP(S)Ph2, cyclopentadienide anion). 

Fn some cases, r-allylpalladium complex formation by retention syn attack) has been observed. The reaction of the cyclic allyiic chloride 33 with Pd(0) affords the 7r-allylpalladium chlorides 34 and 35 by retention or inversion depending on the solvents and Pd species. For example, retention is observed in benzene, THF, or dichloromethane with Pd2(dba)3. However, the complex formation proceeds by inversion in these solvents with Pd(Ph3P)4, whereas in MeCN and DMSO it is always inversion[33]. The syn attack in this case may be due to coordination of Pd to chlorine in 33, because Pd is halophilic. The definite syn attack in complex formation has been observed using stereoche-mically biased substrates. The reaction of the cxoallylic diphenylphosphino-acetate 36 with phenylzinc proceeds smoothly to give 37. The reaction can be explained by complex formation by a syn mechanism[31]. However, these syn attacks are exceptional, and normally anti attack dominates. 

Allylic amine is a less reactive leaving group[7], but the allylic ammonium salts 214 (quaternary ammonium salts) can be used for allylalion(l30,131]. Allylic sulfonium salts are also used for the allylation[130]. The allylic nitrile in the cyclic aminonitrile 215 can be displaced probably via x-allylic complex formation. The possibility of the formation of the dihydropyridinium salts 216 and subsequent conjugate addition are less likelyfl 32],... 

Carboxylate anions are better nucleophiles for allylation. The monoepoxide of cyclopentadiene 343 is attacked by AcOH regio- and stereoselectively via tt-aliylpalladium complex formation to give the m-3,5-disubstituted cyclopen-tene 344[212]. The attacks of both the Pd and the acetoxy anion proceed by inversion (overall retention) to give the cis product. 

Allylic ester rearrangement is catalyzed by both Pd(II) and Pd(0) compounds, but their catalyses are different mechanistically. Allylic rearrangement of allylic acetates takes place by the use of Pd(OAc>2-Ph3P [Pd(0)-phosphine] as a catalyst[492,493]. An equilibrium mixture of 796 and 797 in a ratio of 1.9 1.0 was obtained[494]. The Pd(0)-Ph3P-catalyzed rearrangement is explained by rr-allylpalladium complex formation[495]. 

Conversion of 5-allylthioimidates into /V-allylthioamides is catalyzed by Pd(Il). 2-Allylthiopyridine (820) is converted into the less stable l-allyl-2-thio-pyridone 821 owing to Pd complex formation[509], Claisen rearrangement of 2-(allylthio)pyrimidin-4-(3//)-one (822) affords the A-l-allylation product 823 as the main product rather than the A -3-allylation product 824[510] The smooth rearrangement of the allylic thionobenzoate 825 to the allyl thiolo-benzoate 826 is catalyzed by both PdCl2(PhCN)2 and Pd(Ph3P)4 by different mechanisms[511],... 

The first report on the Pd(II)-promoted Cope rearrangement is the conversion of fw./ra/w-l,5-cyclodecadiene (44) into c/5-l,2-divinylcyclohexane-PdCl2 complex (45) with a stoichiometric amount of PdCl2(PhCN)2 at room temperature. The complex formation is the driving force of this unusual rearrangement [38,39]. A similar transformation of germacrane (l,5-dimethyl-8-isopropyli-dene-/rfflu,/ra j-l,5-cyclodecadiene) takes place[40j. 

Fig. 4. Recognition and preorganization of hosts (receptors) on complex formation (21).

Chemical reactions that the xylenes participate in include (/) migration of the methyl groups, (2) reaction of the methyl groups, (7) reaction of the aromatic ring, and (4) complex formation. 

Complex Formation. AH four Cg aromatic isomers have a strong tendency to form several different types of complexes. Complexes with electrophilic agents ate utilized in xylene separation. The formation of the HE-BF —MX complex is the basis of the Mitsubishi Gas—Chemical Company (MGCC) commercial process for MX recovery, discussed herein. Equimolar complexes of MX and HBr (mp — 77°C) and EB and HBr (mp — 103°C) have been reported (32,33). Similatly, HCl complexes undergo rapid formation and decomposition at —80°C (34). 

Despite numerous efforts, there is no generally accepted theory explaining the causes of stereoregulation in acryflc and methacryflc anionic polymerizations. Complex formation with the cation of the initiator (146) and enoflzation of the active chain end are among the more popular hypotheses (147). Unlike free-radical polymerizations, copolymerizations between acrylates and methacrylates are not observed in anionic polymerizations however, good copolymerizations within each class are reported (148). 

The first quantitative model, which appeared in 1971, also accounted for possible charge-transfer complex formation (45). Deviation from the terminal model for bulk polymerization was shown to be due to antepenultimate effects (46). Mote recent work with numerical computation and C-nmr spectroscopy data on SAN sequence distributions indicates that the penultimate model is the most appropriate for bulk SAN copolymerization (47,48). A kinetic model for azeotropic SAN copolymerization in toluene has been developed that successfully predicts conversion, rate, and average molecular weight for conversions up to 50% (49). 

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