Aquation and Anation

Palladium(ii).—Aquation and Anation. Discussion and controversy continue over the interpretation of the kinetics of the anation and aquation reactions which constitute the equilibria (2). Bekker and Robb report rate constants and activation para- 

Aquation of [PdX(LLL)]+ cations, where LLL = Et dien or its JV -methyl derivative and X = Cl, Br, or I, is catalysed by Hgaq or Cdaq. The kinetic pattern observed corresponds to reversible generation of a dinuclear intermediate whose solvolysis is rate determining  

This appears to be the first kinetic study of catalysis by a soft cation of aquation of a chloro-complex of a square-planar metal centre many analogous examples are of course known for octahedral complexes. Interestingly, whereas Hg + is very effective both in octahedral and in square-planar systems, Cd + is a very feeble catalyst in octahedral systems (c/., for example, aquation of the [ReClJ anion ). Another type of aquation catalysis well established for octahedral complexes is that of acid catalysis of aquation of nitro- and azido-complexes. Kinetics of comparable reactions have now been described for acid-catalysed aquation of [PdX(LLL)]+ cations, where LLL = dien, Et4dien, or the A -methyl derivative of the latter, and X = N3 or NO2, in methanol. These reactions were conducted in the presence of chloride, and the complete rate law established as 

There are thus four significant pathways, both the unprotonated and protonated 

Pearson and M. J. Hynes, Kungl. Tekn. Hogsk. HandL, 1972, 285, 829. 

Exchange of free chloride ion with the chloride trans to the diethyl sulphoxide in the [Pt(EtaSO)Cl3] anion is very rapid. Exchange of free chloride with the cw-chlorides of [Pt(EtaSO)Cl3] is a first-order process whose rate constant is 1.54 X 10 s . This rate of exchange is about one third of that for chloride exchange with [Pt(DMSO)Cl3]. This small difference in exchange rates for the analogous dimethyl sulphoxide and diethyl sulphoxide complexes is attributed to steric effects.  

The breadth of the hydride resonances observed in n.m.r. spectra of compounds frfltn5-[Pt(H)(X)L2], where X = NCS or NCO and L = PEt3 or AsEts, was attributed to quadrupole effects. Current opinion is that this linebroadening is more likely to arise from ligand-exchange processes.  

The mechanism for the displacement of oxalate from [Pt(ox)3] by thiocyanate is complicated. An intermediate anion [Pt(ox)2(SCN)2] , presumably containing unidentate oxalate ligands, has been isolated and characterized as the trihydrate of its potassium salt. Other transient intermediates containing unidentate oxalate (LL), (2)—(4), are proposed, and many rate constants relating to various steps in the postulated reaction sequence have been estimated.  

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Spectrophotometric methods have been used to follow the kinetics of aquation and anation of [RuCl4(H20)2], [RuCl3(H20)3], [RuCl2(H20)4]" " and [RuCl(H20)5] " " over a temperature range 288-318 The aqua-ammine complex [Ru(H20)3(NH3)3] " has been prepared and isolated as the trifluoromethanesulfonate salt7 ... 

Wang F, Chen H, Parsons S, Oswald IDH, Davidson JE, Sadler PJ (2003) Kinetics of aquation and anation of mthenium(II) arene anticancer complexes, acidity and X-ray structures of aqua adducts. Chem Eur J 9 5810-5820... 

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. 

Z>) Reactions of [Rh(NH ),X. The aquation and anation of [Rh(NH3X]" in acidic aqueous solution (equation 105 was first studied by Lamb who used conductance changes to monitor the interconversion of acido and aqua rhodium complexes , for the chloro and bromopentaanimine-rhodium(III) cations. He found that both aquation and anation are first-order in [Rh ], and that the hydrolysis is accelerated by the presence of OH . The forward form of equation (105) (aquation) has since been studied for X = Cl-, Br , l-, >.=56 oHj, SO, NO3" and Nj . Representative results for the aquation of various pentaammine salts are given in Table 25. For aquations of halides or nitrate ions, rate expressions which are first-order in [Rh] and zero-order in [H+] are found. Poe gives a detailed discussion of the thermodynamic, kinetic and intrinsic-kinetic trans effects, and compares the aquation and anation of [Rh(NH3)5X]-" (X = Cl, Br and I) with the [RhfenjjXY]" system. When X = S04, a two-term rate expression was observed... 

Complete volume profiles for the reactions of Eq. (6) have been obtained by separate examinations of the aquation and anation steps. Figure 5.1 shows the results. The activation volumes of -10 to -12cm mor are generally taken to... 

Cobalt(III) forms a wide range of substitution-inert (low-spin d ) complexes whose thermal aquation and anation reactions have been thoroughly studied. These provide useful comparisons for photochemical... 

Cl2]Cl racemizes when heated even though water is not involved in the reaction. An equation-anation pathway has been proposed for the first reaction, but it has not been ascertained whether the aquation and anation reactions are dissociative or associative in character. The role of water in this racemization is possibly due to the high effective concentration of water molecules in the system as a result of their high mobility relative to charged species in the crystal lattice. This gives rise to a large number of encounters between H2O and /-a s-[Cr(en)2Cl2] or a reactive intermediate. 

Hexahalogenoosmium anions [OsXa] " undoubtedly contain osmium in oxidation state 4 -h it is with this currently popular class of complexes that the remaining references in this section deal. Rate constants and activation parameters (A// and A5 ) have been determined in acidic aqueous solution for 13 aquation and anation reactions of the type represented by the forward and reverse processes of the equilibrium of equation (24) ... 

The quantum yields are in the region of 10 for this photoanation reaction. A comparative study of the photoaquation and photoanation reactions has been carried out by photolyzing Cr(NH3)6 in an aqueous solution containing added chloride ion. For a series of solutions covering a chloride ion concentration range from 0-8 M, the total quantum yield is independent of the chloride ion concentration. Aquation and anation therefore only compete with each other for the vacant coordination position in Cr(NH3)i that is formed by the photodissociation of an ammonia molecule. Since the lowest excited quartet state that leads to photodissociation is very short-lived (t < 10" only those molecules present in the... 

Palladium(n).—Aquation and Anation. In aqueous perchloric acid the aquation of [Pd(NH3)4]2+ takes place in four consecutive and kinetically distinct steps, to give ultimately [Pd(OH2>J +. Rate constants decrease steadily as the number of ammonia ligands is reduced. Activation enthalpies for the four steps all lie between 21 and 24kcal mol activation entropies are between + 4 and — 4 cal deg mol . Kinetic parameters for the aquation of [PdClJ and of [PdBrJ " appear from two sources > they are compared in Table 4. Faster aquation rates for [PdBrJ " compared with [PdClJ appear... 

Stereochemical and kinetic studies of the stepwise aquation of the [Cr(CN)6] and [Cr(CN)5(NO)] anions have indicated that the cyano-ligand has a strong rranj -labilizing effect in this type of complex. Hence it seemed an attractive idea to study this postulated effect in the simplest member of this group, the [Cr(OH2)5(CN)] + cation. Unfortunately accurate kinetic studies of substitution reactions of this complex were ruled out by the observation that aquation and anation processes occur at comparable rates.The strong trans effect of iodide complexed to chromium(m) is mentioned in the section on mixed and non-aqueous solvents below. 

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