Unidentate Leaving Groups

Unidentate Leaving Groups.—Rate constants and activation parameters 

Similar examples are afforded by chromium(m)-halide-ammine complexes. Kinetic parameters have been determined for the aquation of [CrBr2(NH3)(OH2)en]+ to [Cr(OH2 4en] + as the kinetics of aquation of the latter are already known, the complete kinetic picture of aquation of the former is now established. Kinetic results have been reported for several other multi-ligand complexes of this type, including [CrBr2(NH3)2(OH2)2l+, cis- and /mnj-[Cra2(NH3)(OH2)3]+ [CrCl(NH3)(OH2)4] +, and [CrCl2(NH3)3(OH2)]+, which last is an intermediate in the aquation of [Cr(Oa)2(NH3)3] in hydrochloric acid.  

Aquation of the nitrito-complexes [Cr(ONO)(NH3)5] + (ref. 100) and [Cr(ONO)(OHa)5] + (ref. 101) is unexpectedly fast. In these cases, aquation takes place without chromium-oxygen bond breaking. Moreover, the aquation of [CrX(OH2)5] , X = Cl or Br, is specifically catalysed by nitrous acid. This is explained by ready formation of [CrX(ONO)(OH2)4]+ as an intermediate, by direct attack of nitrous acid on co-ordinated water again without chromium-oxygen bond fission. The suggestion is supported by the observation that aquation of [CrBr(NHa)5] + is not accelerated by nitrous acid.  

A recent kinetic study confirms the conclusion drawn earlier from product characterisation/ that aquation of 4-pyridiomethylpenta-aquochromium(iii) involves homolysis to hexa-aquochromium(ii) and the +NH C5H4-CH2 radical. There is, however, no evidence for a similar homolytic, or electron transfer [cf. cobalt(iii), previous section] mechanism operating in aquation of azide or of iodide complexes. 

Correlation of kinetic results with d d spectroscopic data has been attempted for a series of chromium(iii) complexes. Spectroscopic data have been expressed in terms of the two-dimensional spectrochemical series , for this should give separated a- and 7r-contributions and thus facilitate comparison with the kinetic results which for chromiumfm) are assumed, as a first approximation, to be a function only of chromium-ligand bond strength.  

Unidentate Leaving Groups.—Studies of complex ions of the type [Co(NH8)s-(solvcnt)] + continue to appear. When the solvent is DMSO, studies of the rate of solvolysis have been made in 71 different aqueous-organic solvent mixtures. The rate constants are independent of ionic strength at constant [CIO ], but show a specific perchlorate-ion dependence. The results are discussed in terms of the h mechanism, and also by using the Bennetto-Caldin model. The effect of solvent variation on the activation parameters is very small. 

Aquation of the [Co(NH3)6(DMSO)] + ion has also been studied at pressures up to l.Skbar (298.2 K, 7=0.1 mol dm ) and the volume of activation found to be —1.7+0.7 cm mol This value of AF is compared with values for related aquation reactions in Table 1. Despite the similarity in values of AF for Co  

Further studies of the [Co(NH3)e(DMSO)] + ion under neutral conditions show that the anation and solvent exchange reactions involve conjugate-base formation and subsequent rapid substitution, probably by reduction to Co. The conjugate- 

A precursor complex containing an Fe—S bond has been detected during the redox reaction between [Co(NH3)s(DMSO)] + and [Fe(CN)5(OH2)] ions. At 298.2 K and 7=0.10 mol H, the rate constant for the aquation of this precursor complex is A =0.106s with A/7 = 20 kcal mol and A5 = 4calK mol . Cleavage of the Co —sulphoxide bond is involved in this aquation step. 

Competition between NO2 and NH3 as leaving groups has been established for the aquation of nitro(ammine)cobalt(iii) complexes in weakly alkaline media. The trans effect of N-bonded NO2 ion is believed to be important here, although the effect is not so marked as with iS-bonded sulphite ion. 

Unidentate Leaving Groups.— There has been considerable interest in the aquation of the [Co(NH3)sX] + cations. Transition enthalpies (AHiy the difference between the enthalpy of the transition state and the enthalpy of the products) have been estimated for aquation of the [Co(NH3)sX] + cations with X = Cl, Br, I, or NO3. For the chloro-, bromo-, and nitrato-complexes. 

Research Inst. Catalysis, Hokkaido Univ., 1972, 20, 34 Chem. Abs., 1973, 78, 48 469x). K. B. Yatsimirskii, Teor. i eksp. Khim., 1972, 8, 617 Chem. Abs., 1973, 78, 34 381a). See pp. 179—180 of Volume 1 and p. 171 of Volume 2 of this Specialist Periodical Report. 

Further evidence confirming the essentially dissociative nature of aquation of complexes [Co(NHg)gX] + is provided by determination of the activation volumes for the compounds with X = Cl, Br, NO3, SO4, or NCS. As reported briefly earlier, these activation volumes correlate with molar volumes of reaction. The present report quotes values for activation volumes extrapolated to zero pressure, amplifies the earlier discussion, and concludes that the aquation mechanism is one of dissociative interchange (/d).  

Whereas considerations of transition enthalpies and of activation volumes have led to evidence useful in confirming the mechanism of aquation in cobalt(m)-ammin6-ligand complexes, an attempt to derive similarly useful information from a comparison of kinetic and solubility parameters proved unsuccessful. No correlation was found between enthalpies of activation for aquation of complexes [Co(NH3)5X] +, with X = F, Cl, Br, I, NCS, N3, or NO2, and the respective enthalpies of solution of their perchlorate salts. In fact it seems impossible to correlate these activation enthalpies with any other apparently reasonable parameter, though it does seem that there is a correlation between activation entropies and the entropies of solvation of the leaving anions.  

In the final references to aquation of complexes [Co(NH3)5X] + the interest is in the reactivity of ion pairs. Rates of aquation of the complexes with X = Cl, Br, or I increase by a factor of two to thi-ee when these cations are ion-paired with dinegative ions such as sulphate, malonate, maleate, or phthalate. The role of ion pairs with uninegative anions in the aquation of 

Cobalt(ill).—Unidentate Leaving Groups. Volumes of activation for the aquation of [Co(CN)5X] ions and for the anation of [Co(CN)5(OH)2] ion are collected in Table 1. The data are entirely consistent with the well established extreme dissociative Z)-mechanism for ligand substitution (see Introduction, ref. 19). 

Co ii-NCS-Hg precursor, in line with the greater affinity of Hg + ion for S-donor ligands.  

The induced aquation of [Co(NHs)6N3] + ion in the presence of nitrous acid and Hg2+ ion has also been investigated in aqueous perchlorate solutions. In this case, the pseudo-first-order rate constant A oi)s = [H+] [NOa ]totai/ ([H+]-f-5[Hg +]), where A and B are constants. The Hg + ion inhibits attack of NO+ ion by competing with H+ ion for the nitrite which is present. In mixed acetonitrile-aqueous solutions, formation of [NgNOal is more complete, fi[Hg +] [H+], and then / obs = [H+ ][N02 ]totai/[Hg +]. For the Hg + ion-induced aquation of [CoCNHaisCl] ion in nitrate media, A obs = (A o+ i[N03 ])-[Hg +], with two pathways postulated involving [CoClHg] +and [CoClHgNOs] precursors. It is concluded from competition experiments that if the five-co-ordinate, [Co(NH3)6] +, intermediate is formed in the reaction of [Co(NH3)6Ns] + with NO+ ion, this intermediate is not formed in either of the Hg + ion-induced pathways.  

This rate expression supports the Z)-mechanism established previously for such reactions  

The greater rranj-effect of SO3 - ions is attributable to a lower value for SH (by 9 1 kcalmol ), partly offset by a less positive value of A5. Using a simple harmonic oscillator model, it is estimated that the Co—N bond must be stretched to 320 pm to achieve a dissociative transition state, and this estimate agrees with previous values calculated from measurements of AF.  

K- mol-. Similar results are reported for the [Co(NH3)4(OH2)2l + ions. A mechanism involving OH and NHg radicals is proposed for the redox step. Dimethyl sulphoxide (DMSO) is a much better leaving group than ammonia, and studies of a variety of reactions involving the [Co(NHa)5(DMSO)] + ion support the expected dissociative interchange or U mechanism  

Both the forward and reverse steps of this reaction have been investigated in aqueous or mixed aqueous-organic solvents. For the forward reaction the rates did 

Physical Chemistry in Organic Solvent Systems , ed. A. K. Covington, Plenum Press, London, 1973, p. 681. 

Vdrhelyi, and K. Szasz-Sata, Acta Ghint. Acad. Set. Hung., 1974, 80, 177. 

Clearer evidence for two isomers when X = Br comes from recent C n.m.r. studies in which the major component is assigned the unexpected axial structure (Br out of the N—Co—N plane) in contrast to the more usual equatorial configuration. Further studies which sometimes involve chelate ring closure are reported for the following systems  

The [(H20)5CrH] ion can be prepared by uv flash photolysis of aqueous chromium(II) perchlorate. Previously it was made by pulse radiolysis. The ion is characterized by a uv peak at 385 nm, and is shortlived owing to its rapid reaction with HsO ion [equation (1)]. At an 

The ki term is dominant with k2 varying with R and R as shown in Table 6.1. For the [(H20)CrCMe2(0H)] ion homolysis of the Cr-C bond is involved, whereas for the other ions a mechanism involving attack by the oxidant at the alcoholic OH group is favored  

With this scheme the electron transfer step is rate limiting and k-2 — kikJk-2. An alternative scheme for Fe involving a rate-limiting ligand substitution process on [Fe(OH)] could not be ruled out. Replacement 

For the inhibition, kobs = cu[02][Cu ] with Me2CH(R) and Me2CHOO(ROO ) acting as chain-carrying intermediates/ Acid hydroly- 

The homolysis rates (Table 6.2) are associated with an 5h1 mechanism, and were obtained using oxidizing scavenger metal ions. The enthalpies of activation are large (20-37 kcal mol ) as expected for such a process. Values of the very small equilibrium constants, Kh, for equilibrium (7) 

Both cw- and /ra -[Co(en)a(NH3)(P04H3)] + aquate by cobalt-oxygen bond fission, but [Co(NHs)5(As04)] has been shown by tracer experiments to aquate by arsenic-oxygen bond fission. [Co(NH3)b(N02)]+, and 

On heating in perchloric acid solution, the complex [Co(NH3)(OH2)6] + disproportionates. With the [Co(OH2)e] produced being reduced by water, the overall reaction is 

The few preliminary kinetic results suggest that this is a second-order reaction.  

Recent examples of investigations of kinetics of aquation of dioximato-complexes [Co(LLH)2X2) include those of anions with LLH2 = dmgHg and X = NO2, NCS,i° or NCSe, and with LLHg = 1,2-cyclopentadione 

Varhelyi, and I. Zsako, /. Inorg. Nuclear Chem., 1970, 32, 3013. 

Studies of alkylpentaaquochromium(III) complexes of the type [(H20)5CrR] continue to be actively investigated. Recent examples include studies of complexes with R = CMe2X (X = CO2H, and 

Acetate ion has been found to assist the cleavage of the Cr-C bond in [(H20)5CrCH20H]  

The electrophilic reactions of [(H20)5CrRf have been investigated in D2O/H2O mixtures. The rate decreases with increasing [D2O], especially when is the electrophile.  

Adenosine-5 -triphosphate (ATP) forms bidentate A- and A- complexes of the type [Cr(H20)4(ATP)] [stmctures (1) and (2)]. Hydrolysis gives predominantly free ATP with very little of the diphosphate, ADP, being formed. The rate constant is 5 x 10 s at 310 K and pH 11. The terdentate ATP complex [Cr(H20)3(ATP)] is also formed, and interestingly this hydrolyzes to give mostly ADP and very little free ATP (fc 5 x 10 s at pH 11 and 310 K). The rate constant is 5000 times as large as the value observed for the alkaline hydrolysis of ATP in the absence of metal ions.  

Several chromium(III) complexes have been made with amino alcohols of the type [Cr(LL3)] [LL = NH2CH2CHRO, where R = H, Me, or NH2CH(Me)CH20 ]. Acidolysis proceeds in three stages, as in equations 

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Chroniium(III).—Unidentate Leaving Groups. Following measurements of AV values for substitution reactions of chromium(iii) by Stranks, Swaddle, and co-workers, there is a general belief that the majority of such reactions proceed by an associative interchange, /a, mechanism. Nevertheless recent evidence suggests that in some cases dissociative, la or D, mechanisms are also possible. For example, the values of A V for the Hg -catalysed aquation of [M(NH3)5X1 + ions (Table 12) are very similar for M = Coi, Rh, or Cr and in line with a... 

Unidentate Leaving Groups.— The question of the detailed mechanism of aquation of chromium(m) complexes, particularly of the [Cr(OH2)5X] + type, has attracted considerable interest and research effort in the past two or three years. Originally it was suggested that the aquation of this type of complex took place by a purely dissociative [5 1 (lim) or D] mechanism, with a [Cr(OHa)5] + transition state or transient intermediate. Later kinetic studies showed that other mechanisms were consistent with kinetic and product data. In particular for iodo-complexes a strong trans labilizing influence of the iodide on the opposite water ligand was consistent with the experimental results the mechanism ... 

The variation of aquation rate with pH of the [Cr(OHa)5(N3)] + cation will reflect both ionization of a proton from co-ordinated water and protonation of co-ordinated azide. The most recent kinetic study of aquation of this complex reports the rate law for aquation in 1—IIM-HCIO4 (see the section on unidentate leaving groups above), where it is the reversible protonation of the azido-ligand which affects rates of aquation. 

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