Anation reaction

The anation reactions of [(H20)5Cr-(alkyl)] complexes are unusually rapid. When a potential carboxylate ligand, RCOJ, and [(H20)5Cr(CH2CN)] are mixed in dilute aqueous acidic solution, there is an initial rapid (minutes) reaction (11). This initial complex (6) can be partially destroyed by the addition 

Coordination of porphine ligands labilizes aqua ligands in the axial positions of Cr(III) complexes, relative to [Cr(NH3)5(OH2)]. The degree of labilization depends on the nature of the porphine and Table 6.7 shows the extent of this effect. In the anation of [Cr(porphine)(OH2)2] , charge plays only a minor role and activation parameters are also similar. The positive values of A V suggest an Id mechanism, after ion pair formation, in contrast to the pentaamine where an /a process is favored. This change in mechanism is attributed to changes in 

Although [Cr(OH2)6] and [Cr(OH)(OH2)5] continue to be the main starting species for anation studies (Table 6.5), there is a growing interest in the use of other aqua ions, such as ci5-[Cr(big)2(OH2)2] or [Cr(ox)2(OH2)2] . In view of the interest in the biochemical role of Cr(III) in the Glucose Tolerance Factor (GTF), several Cr(III) aqua ions have been reacted with insulin and the products characterized after purification by dialysis. The chymotrypsin catalyzed hydrolysis of these Cr(III)-insulin complexes has also been investigated.  

Many anation studies can be described in terms of the mechanism shown in Eqs. (7) and (8), where there is an interchange of an H2O ligand with the anating ligand after the formation of an outer-sphere ion pair. Under such circumstances, the observed rate constant (fcobs) can be described by the expression (9) where a and b are functions of Xqs, and the various protonated forms of the anating ligand and the coordinated aqua ions. 

A comment by Beattie on reaction (7) reconciles two previous conflicting views. Original work described the kinetics as adhering to a 

EtHNC2H4N(Et)C2H2NEtH, or Me2NC2H4NHC2H4NMe2] was followed as a function of Y, T, and P. The second-order rate constants decreased with increasing triL bulk, and were accompanied by an increase in AH. The values of AS and AY were consistent with associative mechanisms in each case. The nucleophilicity order was Y = C Br 1 NJ, in contrast to an earlier report of C N J Br 1 for [Pt(dien)(OH2)f The authors point to precedents for the higher reactivity of azide with palladium, but the rate of anation of the unsubstituted [Pd(dien)(OH2)] is too fast to allow a direct comparison. 

In the formation of related palladium complexes [PdX2(Ph2As-(CH2) AsPh2)] (X = Cl, Br, I, or SCN n = 6-12, or 16), the geometry was always trans when n 9, and mainly trans when n 9, reflecting the common predisposition of Pd towards trans complexes with P and As donors. The shorter-chain diarsines produced polymeric materials, whereas the larger ones led to trans-chelating monomers. 

There continues to be considerable interest in this type of reaction detailed in Eq. (11) and (12), and various deprotonated forms of [Cr(OH2)6] or L are potentially available (Table 6.7), with rate expressions (13) and (14) at constant 

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IS further extended by the replacement of X by other anionic or neutral ligands. The inertness of the compounds makes such substitution reactions slow (taking hours or days to attain equilibrium) and, being therefore amenable to examination by conventional analytical techniques, they have provided a continuing focus for kinetic studies. The forward (aquation) and backward (anation) reactions of the pentaammines ... 

The reaction of alkenylcarbene complexes and alkynes in the presence of Ni(0) leads to cycloheptatriene derivatives in a process which can be considered as a [3C+2S+2S] cycloaddition reaction [125]. As shown in Scheme 77, two molecules of the alkyne and one molecule of the carbene complex are involved in the formation of the cycloheptatriene. This reaction is supposed to proceed through the initial formation of a nickel alkenylcarbene complex. A subsequent double regioselective alkyne insertion produces a new nickel carbene complex, which evolves by an intramolecular cycloprop anation reaction to form a nor-caradiene intermediate. These species easily isomerise to the observed cycloheptatriene derivatives (Scheme 77). 

A number of studies [42,43,52,114] have shown that [TcX6]2 " entered into reduction reactions more readily in hydrohalogenic acids at a concentration of 4-7 M than in concentrated hydrohalogenic acids. It was assumed [9,11,52] that the reduction of [TcC16]2 was promoted by their acidic hydrolysis (anation reaction) (2). 

House, J. E. (1993). "Mechanistic Considerations for Anation Reactions in the Solid State." Coord. Chem. Rev. 128, 175-191. 

The most common reaction exhibited by coordination compounds is ligand substitution. Part of this chapter has been devoted to describing these reactions and the factors that affect their rates. In the solid state, the most common reaction of a coordination compound occurs when the compound is heated and a volatile ligand is driven off. When this occurs, another electron pair donor attaches at the vacant site. The donor may be an anion from outside the coordination sphere or it may be some other ligand that changes bonding mode. When the reaction involves an anion entering the coordination sphere of the metal, the reaction is known as anation. One type of anation reaction that has been extensively studied is illustrated by the equation... 

Table 20.2 Kinetic Data for the Anation Reactions of Aqua Complexes3. ...

In proposed mechanism I, the loss of water from the complex is the rate-determining step, but removal of water from the coordination sphere of the metal ion should be independent of the nature of the anion that is not part of the coordination sphere of the metal ion. On the other hand, if mechanism II is correct, the entry of X into the coordination sphere of the metal would be dependent on the nature of the anion, because different anions would be expected to enter the coordination sphere at different rates. Because there is an observed anion effect, it was concluded that the anation reaction must be an Sn2 process. However, it is not clear how a process can be "second-order" when both the complex cation and the anion are parts of the same formula. As discussed in Chapter 8, it is not always appropriate to try to model reactions in solids by the same kinetic schemes that apply to reactions in solutions. 

Similar reactions have been carried out to produce complexes that contain thiocyanate bridges. The examples given are just a few of the large number of anation reactions. 

The rate of hydrolysis depends strongly on the nature of all three building blocks, i.e. the leaving group, the coordinated arene, and the chelate, and can be varied over several orders of magnitude, opening a time-window of activation. A detailed study of the aquation and the reverse, anation reactions of three [Ru(rj6-arene)Cl(en)](PF6) complexes (arene — bip (10), dha (11), and tha (12)) showed that the rates of aquation... 

As the above discussion indicates, assigning mechanisms to simple anation reactions of transition metal complexes is not simple. The situation becomes even more difficult for a complex enzyme system containing a metal cofactor at an active site. Methods developed to study the kinetics of enzymatic reactions according to the Michaelis-Menten model will be discussed in Section 2.2.4. 

For the low-spin t2g aqua ions [Ru(H20)6]2+, [Rh(H20)6]3+, and [Ir(H20)e]3+ a d-activation mode would a priori be predicted. The approach of a seventh water molecule towards a face or edge of the coordination octahedron is electrostatically disfavored by the filled t2g orbitals which are spread out between the ligands. Rate constants for anation reactions of Cl-, Br-, and I- on [Ru(H20)e]2+ are very similar, indicating identical steps to reach the transition state, namely the dissociation of a water molecule (130). An extension of this study to a large variety of ligands demonstrated clearly that the rate determining... 

The aquated Co(III) ion is a powerful oxidant. The value of E = 1.88 V (p = 0) is independent of Co(III) concentration over a wide range suggesting little dimer formation. It is stable for some hours in solution especially in the presence of Co(II) ions. This permits examination of its reactions. The CoOH " species is believed to be much more reactive than COjq Ref. 208. Both outer sphere and substitution-controlled inner sphere mechanisms are displayed. As water in the Co(H20) ion is replaced by NHj the lability of the coordinated water is reduced. The cobalt(III) complexes which have been so well characterized by Werner are thus the most widely chosen substrates for investigating substitution behavior. This includes proton exchange in coordinated ammines, and all types of substitution reactions (Chap. 4) as well as stereochemical change (Table 7.8). The CoNjX" entity has featured widely in substitution investigations. There are extensive data for anation reactions of... 

NMR spectra, 20 347 structural parameters, 20 297, 299 structure of, 20 300 chalcogenide halides, 23 381-382 structure, 23 382 complexes, 6 2 anation reactions, 34 225 with bipy and phen, 12 185-187 5-coordinate, 34 229, 234, 236-238 with divalent tin, 11 119 with germanium, 11 107-108, 112 homoleptic, 2,2 -bipyridine, 34 28-30 with olefins, 12 311-316 sulfoxides, 24 166 carbonylation reactions, 24 143 and catalyzed decomposition of H2O2, 24 150-151... 

On the general question of the anation reaction of the aquo complex by various anions, there is a little more information. In particular, of course, there are a few cases where one may identify rates of the same sort that are discussed in the paper on nonaqueous systems-i.e., rates of interchange between outer and inner sphere ligands. 

In weakly acidic solutions the various anation reactions are not pH dependent. However, when formation of Co(CN)50H "3 becomes appreciable in alkaline solution, there is a marked decrease in rate. The anation reaction of N3"" was studied in some detail in this pH region. Typical results are given in Table I. 

Before concluding the discussion of the anation reaction, some consideration should be given to alternative formulations of the reaction mechanism. In parti-... 

The aquation of the various CoCCN -3 complexes must occur by a reaction path which is merely the reverse of that followed in the anation. If the proposed mechanism for the anation reaction is valid, the reverse of Reactions 1 and 2 may be used to describe the equation. In any given experiment the rate of approach to equilibrium may be characterized by a first-order rate constant k which is related to the other kinetic parameters by Equation 3. When krfa/kz k (X ), as it is in... 

The aquation reactions of the other complexes do not proceed to completion, even in the absence of added X . Under these conditions two alternative methods may be used to evaluate k. In the first method k is obtained from a study of the initial rate of aquation in the time viterval when the anation reaction may be neglected. In the second method the aquation is studied in alkaline solution where the formation of CoCCN OH-3 tends to drive the reaction to completion. 

The rapid anation reaction observed in the presence of I3 implies that 12 should be a very efficient catalyst for the aquation of Co(CN)5r-3. Qualitative observations confirm this prediction, but a careful study of the catalyzed aquation has not yet been completed. A study of the catalysis by other halogen molecules and Lewis acids will also be undertaken in future work. 

Just the other day I was reviewing the fine paper by Dr. Sutin and his coworker on the anation reactions of the ferric ion, where the accelerating effect of... 

The isomers cis- and trans-[Cr(CN)2(NH3)4]+ as well as m-[Cr(CN)(NH3)4DMS0](G04)2 have also been synthesized by the anation reactions shown in Scheme 29.325 While anation in DMSO is accompanied by stereochemical change, the reaction in H20 is stereoretentive. The dicyano complexes undergo H+-assisted thermal aquation, involving the successive loss of the CN- ligands. The trans complex is about ten-fold more reactive in the first step than the cis, an observation attributed to the trans labilizing effect of CN-. 

Because of the inertness of Co(III) and Cr(III) complexes, their substitution reactions were the first among those of octahedral complexes to be extensively studied. Most evidence supports the fd mechanism for substitution in Co(fll) complexes. First, there is little dependence of reaction rates on the nature of the incoming ligand, if bond making were of significant importance, the opposite would be expected. Data are presented in Table 13.4 for the anation reaction of penta-ammineaquacobaltdll) ... 

The most obvious examples are in the preparation of octahedral Co(III) complexes. The most readily-available cobalt compounds are Co(II) salts in the presence of suitable ligands - usually N-donors - these are oxidised to give Co(III) complexes by air or hydrogen peroxide. A few such easily-prepared complexes open up pathways to the vast number of known octahedral Co(III) complexes via substitution reactions. For example, [Co(NH3)5(H20)]CI3 is readily converted into [Co(NH3)5X]Cl2 via anation reactions of the type discussed in Section 9.5, and salts containing the [Co(NH3)4(C03)]+ ion (where the carbonate is bidentate, taking up two cis positions) are useful for the formation of cw-[Co(NH3)4X2]+. 

Besides the 1,3-dipolar cycloaddition of azomethine ylides to C60, the Bingel cycloprop anation reaction is widely used for regioselective functionalization of fullerenes. In principle, this versatile modification involves the generation of carbon nucleophiles from a-halo esters and their subsequent addition to C60 [19]. The addition takes place exclusively on double bonds between two six-membered rings of the fullerene skeleton, yielding methanofullerenes. As shown in Scheme 2, addition of diethylbromomalonate to C60, in the presence of an auxiliary base... 

The hw(l,2-diaminoethane) complexes are more reactive, and their aquation and anation reactions have been widely studied (Scheme 14). The cw-[RhX2(en)2]+ ion may be resolved, and (- -)-[Rh(ox)(en)2]+ has been assigned the A configuration. The substitution reactions of the X2 ligands proceed with retention of configuration. 

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