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

Abstract

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.

Synthesis and kinetics of formation and aquation of cis-bis(oxalato)bis(dimethylformamide)chromate(III)

“…Since there were two ligands being replaced by two water molecules, it was assumed that consecutive reactions were occurring (eq 2 and 3). It can be k¡ dJ-Cr(C204)2(HC0N(CH3)2)2" + H20 -*m-Cr(C204)2(HC0N(CH3)2)(0H2)" +HCON(CH3)2 (2) cis-Cr(C204)2(HC0N(CH3)2)(0H2)" + H20 m-Cr(C204)2(0H2)2" + HCON(CH3)2 (3) shown that for a system of this type that follows pseudofirst-order kinetics eq 4 is valid.1 A is the absorbance at time A -A" = axe~k'1 + a2e'klt (4) t and A" is the absorbance at infinite time (10 half-lives). The constants ax and a2 are defined by eq 5 and 6, where l is the graphically for kx and k2.x The results as well as the leastsquares calculated values of the activation parameters are in Table I.…”

“…The anation reactions of Co(III), Rh(III), and Cr(III) pentaammine complexes have been extensively studied for a wide range of organic and inorganic entering ligands in aqueous medium.4"21 During the anation process, the ammonia ligands are tightly bound to the metal ion22 and, in most cases,20 are substitution inert. These reactions are generally accepted5 to proceed according to an interchange mechanism:7•23 M(NH3)5OH23+ + X" |M(NH3)50H23+,X1 (1) {M(NH3)5OH23+,X"j M(NH3)5x2+ + h2o (2) where M = Co(III), Rh(III), or Cr(III) and X = Cl, Br, NCS, CH3COO, etc. Burnett18 suggested that ion triplets also participate in the above mechanism at high concentrations of X", although this suggestion was criticized in later work.24…”

<b>Kinetics of substitution of cis-bis(malonato)diaquochromate(III) with glycine, DL-alanine and DL-phenylalanine in alkaline medium</b>

“…Krishnamuthy [1] has reviewed the substitution reactions of the oxalate complexes of chromium(III). However, very few papers have been published on the kinetics of Cr(C 2 O 4 ) 2 X 2 nmoiety; where X is any monodentate ligand other than water [2][3][4][5][6].…”

Kinetics of substitution of cis-bis(malonato)diaquochromate(III) with glycine, DL-alanine and DL-phenylalanine

“…The substitution reactions of the oxalato complexes of chromium(III) have been reviewed by Krishnamurthy [1]. However, very few papers have been published on the kinetics of Cr(C 2 O 4 ) 2 X 2 nmoiety; where X is any monodentate ligand other than water [2][3][4][5][6]. Amino acids are organic molecules that constitute a most important part of biological structure and body chemistry.…”