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

Abstract

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.

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

“…Similar catalytic effects of nitrate ions were reported for the trans to cis isomerization of Cr(C3H204)2(H20)2",2 the anation of m-Cr(C204)2-(H20)2" by oxalate,3,4 and the substitution of Cr(C204)2-(NH2CH2COO)2" by oxalate.5 However, this is not a general tendency since no specific catalytic effect was observed for the anation of cii-Cr(C204)(en)(H20)2+ by oxalate. 11 Very similar effects were recently reported for the anation of cij-Co(en)2(H20)23+, a-cw-Co(edda)(H20)2+ (edda = ethylenediaminediacetate), and Co(NH3)4(H20)23+ by oxalic acid.6"8…”

“…The broad resonances (Ai<1(,2 = 150 Hz) are attributable to the large quadrupolar moments of arsenic and antimony rather than any ligand-exchange process, which would be expected to produce a much wider signal. The compounds [M-(SbPh3)4], M = Pd, Ni,14 do not exhibit exchange spectra at room (11) Browning, J.; Empsall, H. D.; Green, M.; Stone, F. G. A. J. Chem. Soc., Dalton Trans.…”