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Journal ArticleDOI

Mechanism of metal ion promoted dissociation of some oxalato complexes of chromium(iii)

01 Mar 1984-Journal of Coordination Chemistry (Taylor & Francis Group)-Vol. 13, Iss: 2, pp 123-129
TL;DR: In this article, the second-order rate constant kM for a particular substrate complex and different promoter metal ions of same charge varies linearly with log KM-ox, which is the formation constant of the mono-oxalato complex of the catalysing metal ion Mn+.
Abstract: Dissociation of oxalato complexes of chromium (III) of the type [Cr(ox)n (OH2)6–2n](3–2n)+ into [Cr(ox)n-1 (OH2)8–2n](5–2n)+ (where n = 1, 3) promoted by different metal ions of the first transition series in acid perchlorate media has been investigated spectrophotometrically. In comparable cases the order of the catalytic activity as measured by the second-order rate constant kM, follows the sequence Fe3+ >> Cu2+ > Ni2+ > Zn2+ > Co2+ > Mn2+. Log kM for a particular substrate complex and different promoter metal ions of same charge varies linearly with log KM-ox, KM-ox being the formation constant of the mono-oxalato complex of the catalysing metal ion Mn+. The value of kH+ for the acid catalysed reaction occupies a position intermediate between kCo2+ and kMn2+ and even for the positively charged complex [Cr(ox) (OH2)4]+, Fe3+ is a far better catalyst than H+, indicating the importance of specific bonding between the promoter and the substrate complex in the intermediate step. These conclusions a...
Citations
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Journal ArticleDOI
TL;DR: In this paper, two new chromium-III-nicotinate complexes, cis-[Cr(C2O4)2(O-nic)(H2O)]− and cis[Cr(c2O 4]2(N-nic), were obtained and characterized in solution (where O-nic = O-bonded and N-nic=N-bunded nicotinic acid).
Abstract: Two new chromium(III)–nicotinate complexes, cis-[Cr(C2O4)2(O-nic)(H2O)]− and cis-[Cr(C2O4)2(N-nic)(H2O)]−, were obtained and characterized in solution (where O-nic=O-bonded and N-nic=N-bonded nicotinic acid). The kinetics of nicotinate ligand liberation were studied spectrophotometrically in the 0.1–1.0 m HClO4 range, at I=1.0 m. The rate equations were determined and a mechanism is proposed. The rate of Cr–O bond breaking is [H+] dependent: kobs=kHQH[H+], where kH is the acid-catalyzed rate constant and QH is the protonation constant of the nonbonded oxygen atom in the O-coordinated ligand. The Cr–N bond breaking proceeds via two paths: spontaneous and acid-catalyzed; kobs=k0 + kHQH[H+], where k0 and kH are the spontaneous and acid catalyzed rate constants and QH is the protonation constant of the carboxylic group in the N-bonded nicotinic acid. The results demonstrate by comparison that Cr–N bond breaking is a much slower process than Cr–O bond fission.

9 citations

Journal ArticleDOI
TL;DR: In this paper, two [Cr(C2O4)2(AB)]2− type complexes were obtained from the reaction of the AB ligand with the IC ligand, respectively via FeIII-induced substitution of the oxalato ligand.
Abstract: Two [Cr(C2O4)2(AB)]2− type complexes, obtained from the reaction of cis-[Cr(C2O4)2(H2O)2]− with the AB ligand, [AB = picolinic (pyac) or 2-pyridine-ethanoic acid (pyeac) anions], were converted into [Cr(C2O4)(pyac)(H2O)2]0 and [Cr(C2O4)(pyeac)(H2O)2]0 compounds, respectively via FeIII-induced substitution of the oxalato ligand. The aquation products were separated chromatographically and their spectral characteristics and acid dissociation constants determined. The kinetics of the oxalato ligand substitution were studied with a 10–40 fold excess of FeIII over [CrIII] at [H+] = 0.2 M and at constant ionic strength 1.0 M (Na+, H+, Fe3+, ClO−4). The reaction rate law is of the form: r = kobs[CrIII], where kobs = kQ[FeIII]/(1 + Q[FeIII]). The first-order rate constants (k), preequilibria quotients (Q) and activation parameters derived from the k values have been determined. The reaction mechanism is discussed in terms of a Lewis acid catalyzed (induced) ligand substitution.

6 citations

Journal ArticleDOI
TL;DR: The effect of pH on the complex reactivity was rationalized based on proposed mechanisms and the kinetics of two reaction stages, namely the chelate ring opening and the ligand liberation, were studied spectrophotometrically.
Abstract: Acid- and base-catalyzed hydrolysis of [Cr(ampy)(ox)2]−, where ampy = 2-(aminomethyl)pyridine, leads to successive dissociation of the ligands via concurrent reaction paths, whereas at pH 1–9 only ampy is liberated as a result of spontaneous processes. The first ligand dissociation proceeds via aqua intermediates with one-end bonded ampy (1) or ox ligands (2), respectively, which in alkaline media undergo rapid deprotonation to give the appropriate hydroxo-forms. The kinetics of two reaction stages, namely the chelate ring opening and the ligand liberation, were studied spectrophotometrically. In acidic media, the first stage is much faster than the second, whereas in alkaline solutions, both the stages are characterized by similar rate constants. The dependences of k obs on [H+] are as follows: k obs1,H = a 1 + b 1/[H+], k obs2,H = a 2 + b 2[H+]. At pH > 13, rate constants k obs1,OH and k obs2,OH are [OH−] independent. The effect of pH on the complex reactivity was rationalized based on proposed mechanisms.

5 citations

References
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Journal ArticleDOI
TL;DR: In this article, the determination of the velocity constant of a unimolecular reaction is discussed. But the authors focus only on the case where the reaction can be seen as a single particle.
Abstract: (1926). XLVI. On the determination of the velocity constant of a unimolecular reaction. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science: Vol. 2, No. 9, pp. 538-543.

513 citations

Book
02 Jan 1954
TL;DR: Experimental inorganic chemistry, Experimental Inorganic Chemistry (EIC), Experimental inorganic Chemistry, Experimental Chemistry (EC), this paper, Experimental Chemical Engineering (ECE), Experimental Chemical Evolution (ECE),
Abstract: Experimental inorganic chemistry , Experimental inorganic chemistry , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

212 citations

Journal ArticleDOI
TL;DR: In this article, the kinetics of dissociation of cis-Cr(C2O4)2(H2O)2− to Cr(C 2O4)(H 2O)4+ in perchloric acid medium has been studied under different conditions.

29 citations