The reactivity of co-ordinated oxalate. Part V. Oxygen-18 exchange studies on the cis-diaquobisoxalatochromate(III) anion

The kinetics of isomerization and hydrolysis reactions of some anionic complexes of chromium(III) have been investigated.The results obtained for the trans-to-cis isomerizations of Cr(ox)2(OH2)2~, Cr(ox)2(OH2)OH2-, and Cr(ox)2(OH)23are interpreted in terms of one-ended dissociation of the oxalato ligand. In contrast, the similar malonato series reveals a breakdown in the kinetic pattern, which suggests a change in mechanism for the aquohydroxo and diaquo forms involving the primary dissociation of a water molecule. The hydrolysis of trans-Cr(ox)2(OAc)23occurs at a rate independent of pH in the range pH 2-11, with retention of geometrical configuration. The pH ranges related to the hydrolysis of the aquoacetato or the hydroxoacetato complexes are below 6.8 and above 10.8, respectively; the hydrolysis of the former is acid catalyzed. The final product of all these reactions is cA-Cr(ox)2(OH2)2~o r its conjugate bases. The mechanism of the hydrolysis is discussed in the light of the isomerization behavior of the related species above, of the activation parameters, and, in part, of the stereochemical results. A common feature is the tendency to a dissociation mechanism as a function of the overall negative charge of the complex.

Section: Resultsmentioning

confidence: 99%

“…The values of the dielectric constants of the solvents were taken from the literature. 10 The first-order rate constant, 104& (sec-1) at 25°, were as follows (per cent MeOH in parentheses): trans-Ct(ox)i-(OH2)2~, 4.19 (0%), 3.0 (15%), 1.90 (30%), 0.75 (60%), 0.23 (90%); Irans-Cr (mal )2 (OH2 )i~, 0.018 (0%), 0.07 (90% at 46.3°).…”

Section: Methodsmentioning

confidence: 99%

Structure and reactivity in octahedral complexes. XIII. Trans-to-cis isomerizations and hydrolysis reactions of some chromium(III) anionic complexes

Casula¹,

Illuminati²,

Ortaggi³

1972

“…Solution pH should decrease by less than 0.03 pH units in solutions of initial pH >4.00. species generalizes extremely well to the anation reaction of di-Cr(ox)(en)(H20)2+. This mechanism, summarized by eq [9][10][11][12][13][14][15][16], requires association of cí'í-Cr(ox)(en)(H20)2+ with H2C204 áiH+ + HC204- (9) °Parameters of fit to eq 17; KB = K2KC/K2'.…”

Section: Discussionmentioning

confidence: 99%

Anation of the cis-diaqua(ethylenediamine)(oxalato)chromium(III) complex ion by oxalate species

Kallen¹,

Stevens²,

Hammond³

1979

At pH 5 and 35.0 OC the C~~-C~(C~O~)(NH~CH~CH~NH~)(H~O)~+ complex ion undergoes anation by oxalate species in aqueous solution to produce Cr(C204)z(NH2CH2CHzNH2)-and small quantities of Cr(C204)2(NH2CH2CH2NH3)(H20). The products of the anation reaction are unstable in this pH region and in turn undergo a series of reactions which leads ultimately to C T ( C~O~)~~-.The kinetics of the initial anation reaction have been investigated at temperatures from 20.0 to 40.0 OC, at pH values from 2.00 to 5.00, at formal oxalate concentrations from 0.050 to 0.300 F and at ionic strengths of 0.80 and 1.00 M in potassium nitrate media. Observed pseudo-first-order rate constants exhibit a complex hydrogen ion dependence and a mass law retarded first-order dependence on the formal oxalate concentration. These rate constants have been interpreted in terms of a mechanism in which ion pair formation between HCzO4-( K B ) , C2042-(Kc) and the parent complex precede a common ligand interchange step ( k , ) . In general, k o~ = klKc[H']([H'] + K2/)[C20t-]/{K2/([H'] + Kal) + Kc[Hf]([Ht] + K2/)[C204z-]), where KZ/ is the acid dissociation constant of an associated hydrogen oxalate ion and Kal is the first acid dissociation constant of the aqua ligands of the complex ion. At 25.0 OC and an ionic strength of 0.80 M, pKal = 5.92, K2/ = (7.25 A 0.15) X M, KB = 1.35 M-], Kc = 4.70 f 0.08 M-l, and k l = (1.44 i 0.05) X lO-'s-I.In the stated temperature interval and at an ionic strength of 0.80 M, the activation parameters of kl were found to be AIP = 18.1 f 0.4 kcal mol-' and AS* = -10.7 i 1.4 cal mol-' K-I. A two-step oxalate anation mechanism has been described for this complex ion and other oxalato complexes of chromium(II1). The mechanism is initiated by one-ended dissociation and isomerization of an oxalato-0,O'ligand to the oxalato-O,O bonding mode. Associative interchange of an ion-paired oxalate species for an aqua ligand is proposed to occur in the second step as a direct consequence of ligand isomerization. This mechanism successfully explains differences in the nature of the activation parameters for oxalate anation of oxalato complexes and other complexes of chromium(II1). It also generalizes to provide a water exchange mechanism for oxalato complexes of chromium(II1) which does not limit the anation rate.

“…At 50°C and an ionic strength of 1.00 M, L• = 3.6 X 10™3 sec™1, K = 0.85 M™1 for the hydrogen oxalate ion, and K = 1.8 M~x for the oxalate ion. We propose the following, alternate, steady-state mechanism to bring a degree of similarity to the seemingly different systems c¿s-Cr(C204)2(H20)2-|=iA; d[A]/dr = 0 (12) A + L Cr(C204)2(L)(H20)" + H20 (13) Cr(C204)2(L)(H20)' Cr(C204)33" + H20 (14) Our mechanism identifies kw as ki and K as h/ke. In the context of our mechanism, the parameter K describes the fate of intermediate A in the presence of HC2O4™ or C2O42™ and has magnitudes which are consistent with the need for attack by an unprotonated carboxylate group of the entering ligand.…”

Section: Resultsmentioning

confidence: 99%

Reactions of the trans- and cis-diaquobis(oxalato)chromate(III) ions in solutions which contain oxalic acid

Kallen¹

1976

When t/'iZZM-Cr(C204)2(H20)2-is added to acidic solutions that contain H2C2O2 and HC2O4-, rapid isomerization of the trans complex occurs. This step is followed by the slower anation of the resulting cis complex by oxalate species. Anation proceeds to an equilibrium mixture of species described by the equilibrium quotient Ki = [Cr(C2CU)33-] [H+]2/ [cis-Cr(C204)2(H20)2-][H2C204]. At 50°C and µ = 1.00 M, maintained with potassium nitrate, Ks = 0.890 ± 0.012 M. The kinetics of the two-step reaction at µ = 1.00 M, between 35 and 55°C, are reported. Both steps of the reaction obey simple first-order kinetics. Observed pseudo-first-order rate constants for the trans-cis isomerization in sodium perchlorate and potassium nitrate were fit to the form /cobsd = kn2o + £m[M+] + &h[H+], where M is sodium or potassium. Activation enthalpies for the various constants are 15.1 ± 0.5 (fcNa), 14.7 ± 0.8 (kt0, and 18.5 ± 0.3 (ku) kcal/mol. Activation entropies are -23.9 ± 1.6 (kNa), -26.0 ± 2.6 (/ck), and -8.5 ± 0.8 (kn) cal/(mol deg). Data interpretations are based on AH* = 17.9 kcal/mol and AS* = -14.7 ca!/(mole deg) for Zch20. Hydrogen ion and ligand dependencies of the observed pseudo-first-order anation rate constants are medium-dependent and slower rates are observed in perchlorate media. In perchlorate media, Zcobsd = /cb[HC204"] + Zc4[H+], while in nitrate media, Acobsd = {/ci[H2C2O4] + (/:2/:4//c3)[H+]2|/|l + (k2//£3)[H+]|. At 50°C in potassium nitrate, k\ = 2.84 X 10-2 AH sec-1, ki/ki = 73.8 ± 11.3 M-1, and ¿4 = 4.38 X 10-4 AH sec-1. Activation enthalpies are 20.7 ± 0.4 and 21.9 ± 0.4 kcal/mol, and activation entropies are -1.9 ± 1.0 and -6.5 ± 1.4 cal/(mol deg), for k] and /c4, respectively. A steady-state mechanism, which describes k 1 and ki as anation steps and ki and ¿4 as aquation steps, is proposed for the anation reaction in both media. This mechanism identifies kb as a composite constant equal to ktks/kiKt, where K\ is the first dissociation constant of oxalic acid. The anation rate constant, k\, is limited by kn for the trans-cis isomerization. Other mechanistic similarities between the isomerization process and aquation and anation processes are described, and a new conformation for the bidentate oxalate ligand is proposed as a precursor to each process.