Anation of the cis -Diaquabis(oxalato)chromate(III) Complex Ion by Oxalate Species

TLDR: In the presence of the nitrate ion, direct chelation of the binoxalato-0 ligand of trans-Cr(C204)2(0COC02H)(H20)2- (k3l) competes favorably with aquation (kZl) to fully activate this anation pathway as discussed by the authors.

Abstract: in the absence of catalysis. In the presence of the nitrate ion, direct chelation of the binoxalato-0 ligand of trans-Cr(C204)2(0COC02H)(H20)2- (k3l’) competes favorably with aquation (kZl’) to fully activate this anation pathway. At 50.0 OC and an ionic strength of 1.20 M, kl = (4.02 f 0.06) X lo-) s-I, kll/kz = 0.50 i 0.04 M-l, kll‘/k2 = 0.26 f 0.04 M-I, k12/k2 = 1.56 i 0.10 M-I, and k3l’/k21’ = 3.0 * 0.5 M-I. The activation parameters of kl were found to be AH* = 18.0 EL: 0.2 kcal mol-l and AS* = -14.0 f 0.5 cal mol-l K-I at an ionic strength of 1.20 M. The reaction mechanism has been extended to include data previously reported near pH 1. The combined data show that nitrate-catalyzed anation proceeding through trans-Cr(C204)2(0COC02H)(H20)2- is a process of high activation enthalpy and positive activation entropy. In contrast, substitution proceeding through cis intermediates has a low activation enthalpy and a negative activation entropy.