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

Abstract

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.

Competitive pathways for the oxalate substitution reaction of the (glycinato-N,O)bis(oxalato)chromate(III) ion

“…These constants have been calculated with the use of the rate laws and rate data specific to potassium nitrate and sodium perchlorate media over the 35-55 °C temperature range. 3 The reaction steps and rate laws for the anation process are given in eq 5-9.6 The close correspondence of kb to the rate «>Cr(C204)2(H20)2-+ H2C204 Cr(C204)2(02CC02H)(H20)2" + H3G+ (5) Cr(C204)2(02CC02H)(H20)2~5 =t Cr(C204)33" + H30+ (Glycinato-7V,0)bis(oxalato)chromate(III) Ion…”

“…8, 1979 2155 differ by a factor of 2 as Kallen's data interpretation indicates. 3 Finally, the rate constant ratio k2/k3 is the same in potassium and sodium nitrate and is sufficiently large that the rate law will collapse to the simpler form given by eq 9 when the nitrate concentration is reduced to zero at modest hydrogen ion concentrations. This collapse successfully explains the existence of different rate laws for the anation process in nitrate and perchlorate media.…”

“…This compares almost perfectly with the expression kb = {1.91 X 10"3• [HC204~] + 2.00 X l(T4[H+]j s~* interpolated from Kallen's data for sodium perchlorate media. 3 Activation parameters for the rate constants of eq 17 are included in Table III. Caution must be exercised in interpretation of these parameters because they may represent the result of "averaging" similar, but not identical, nitratedependent and nitrate-independent pathways.…”

“…The anation of tis-Cr^C^CI^O^by oxalate species has also been examined in acidic solution. 3 In this case monodentate substituion of a hydrogen oxalate ligand was found to be competitive with the rapid trans-cis isomerization reaction of Cr(C204)2(H20)2™. However, complete substitution of an oxalato-0,0' ligand was severely limited by the chelation process.…”

Preparation, characterization, and properties of platinum(0) complexes with tri-n-butyl phosphite, tri-n-butylarsine, and tri-n-butylstibine

“…Found: C, 43.39; H, 5.33; N, 6.38. 'H NMR (CDC13): 6 7.00 (AB quartet, JKB = 8.3 Hz, 4 H, C6ff4CH3), 3.93-3.50 (m, 8 H, CH2CH3), 3.87 (s, 3 H, DMAC(Cf/3)), 3.86 (s, 3 H, DMAC-(CH})), 2.27 (s, 3 H, C6H4CH3), 12 H,CH2Ctf3). IR: 1819 (m), 1802 (s), 1790 (s), 1512 (sh), 1500 (s), 1436 (s), 1357 (w), 1301 (w), 1276 (m), 1238 (w), 1211 (s), 1175 (w), 1152 (m), 1089 (w), 1079 (w), 1036 (w), 1019 (w), 1008 (w), 983 (w), 916 (w), 902 (w), 852 (w), 837 (w), 826 (w), 811 (w), 782 (w), 735 (m), 603 (w), 435 (w) cm"1.…”