The Cr(II) reduction of pentane-2,4-dionatobis(ethylenediamine)cobalt(III). An apparent failure of the product criterion for assigning inner-sphere mechanisms

Philip Acott, Ghazala Ali, Nita Lewis

Research output: Contribution to journalArticle

4 Citations (Scopus)

Abstract

The Cr(II) reduction of [Co(bipy)2(ptdn)2+ (bipy = 2,2t́-bipyridine and ptdn = pentane-2,4-dione) proceeded too rapidly to be detected by the stopped-flow method and a lower limit of 3 × 104 M-1 s-1 (μ = 0.10 M (LiClO4)) and [H+] = 0.030 M was calculated for this reaction. Product studies established that both [Cr(bipy)(OH2)4]3+ and [Cr(bipy)(ptdn)(OH2)2]2+ were formed. The ration of products obtained was strongly dependent on the mixing technique used. Under conditions of excess chromium(II), more than the stoichiometric amount of Cr(III) products were isolated, indicating that some degree of substitution was occurring. Blank experiments established that the products obtained could have arisen from an outer-sphere reaction followed by capture of the free ligands by Cr(II). A synergistic effect between the acac and bipy ligands appears to be operating. Two reactions were observed on the stopped-flow instrument. Both are ascribed to ring-closure of bipy on Cr(III). The reaction to give [Cr(bipy)(OH2)4]3+ was characterised by k25 °C = 5.06 × 10-1 s-1 with activation parameters ΔH = 9.9 ± 0.3 kcal/mol, and ΔS = -26.8 ± 0.9 e.u. The second reaction was ascribed to the formation of [Cr(bipy)(ptdn)(OH2)2]2+ from [Cr(bipy)(ptdn)(OH2)3]2+ and had k25 °C = 5.83 × 10-2 s-2; ΔH = 4.93 ± 0.06 kcal/mol, ΔS = 47.6 ± 0.2 e.u. The final reaction was attributed to breakdown of the Cr(III) products. It occured very slowly with k25 °C = 4.6 × 10-6 s-1 (all values quoted at μ = 0.10 M (LiClO4) and [H+] = 0.030 M).

Original languageEnglish
Pages (from-to)169-176
Number of pages8
JournalInorganica Chimica Acta
Volume99
Issue number2
DOIs
StatePublished - May 1 1985
Externally publishedYes

Fingerprint

ethylenediamine
pentanes
Cobalt
cobalt
Ligands
Chromium
products
Substitution reactions
Chemical activation
rations
ligands
Experiments
blanks
Dione
closures
lithium perchlorate
pentane
chromium
breakdown

ASJC Scopus subject areas

  • Biochemistry
  • Inorganic Chemistry
  • Physical and Theoretical Chemistry
  • Materials Chemistry

Cite this

@article{79668728be8244da88681fe89dbcb74f,
title = "The Cr(II) reduction of pentane-2,4-dionatobis(ethylenediamine)cobalt(III). An apparent failure of the product criterion for assigning inner-sphere mechanisms",
abstract = "The Cr(II) reduction of [Co(bipy)2(ptdn)2+ (bipy = 2,2t́-bipyridine and ptdn = pentane-2,4-dione) proceeded too rapidly to be detected by the stopped-flow method and a lower limit of 3 × 104 M-1 s-1 (μ = 0.10 M (LiClO4)) and [H+] = 0.030 M was calculated for this reaction. Product studies established that both [Cr(bipy)(OH2)4]3+ and [Cr(bipy)(ptdn)(OH2)2]2+ were formed. The ration of products obtained was strongly dependent on the mixing technique used. Under conditions of excess chromium(II), more than the stoichiometric amount of Cr(III) products were isolated, indicating that some degree of substitution was occurring. Blank experiments established that the products obtained could have arisen from an outer-sphere reaction followed by capture of the free ligands by Cr(II). A synergistic effect between the acac and bipy ligands appears to be operating. Two reactions were observed on the stopped-flow instrument. Both are ascribed to ring-closure of bipy on Cr(III). The reaction to give [Cr(bipy)(OH2)4]3+ was characterised by k25 °C = 5.06 × 10-1 s-1 with activation parameters ΔH≠ = 9.9 ± 0.3 kcal/mol, and ΔS≠ = -26.8 ± 0.9 e.u. The second reaction was ascribed to the formation of [Cr(bipy)(ptdn)(OH2)2]2+ from [Cr(bipy)(ptdn)(OH2)3]2+ and had k25 °C = 5.83 × 10-2 s-2; ΔH≠ = 4.93 ± 0.06 kcal/mol, ΔS≠ = 47.6 ± 0.2 e.u. The final reaction was attributed to breakdown of the Cr(III) products. It occured very slowly with k25 °C = 4.6 × 10-6 s-1 (all values quoted at μ = 0.10 M (LiClO4) and [H+] = 0.030 M).",
author = "Philip Acott and Ghazala Ali and Nita Lewis",
year = "1985",
month = "5",
day = "1",
doi = "10.1016/S0020-1693(00)87964-5",
language = "English",
volume = "99",
pages = "169--176",
journal = "Inorganica Chimica Acta",
issn = "0020-1693",
publisher = "Elsevier BV",
number = "2",

}

TY - JOUR

T1 - The Cr(II) reduction of pentane-2,4-dionatobis(ethylenediamine)cobalt(III). An apparent failure of the product criterion for assigning inner-sphere mechanisms

AU - Acott, Philip

AU - Ali, Ghazala

AU - Lewis, Nita

PY - 1985/5/1

Y1 - 1985/5/1

N2 - The Cr(II) reduction of [Co(bipy)2(ptdn)2+ (bipy = 2,2t́-bipyridine and ptdn = pentane-2,4-dione) proceeded too rapidly to be detected by the stopped-flow method and a lower limit of 3 × 104 M-1 s-1 (μ = 0.10 M (LiClO4)) and [H+] = 0.030 M was calculated for this reaction. Product studies established that both [Cr(bipy)(OH2)4]3+ and [Cr(bipy)(ptdn)(OH2)2]2+ were formed. The ration of products obtained was strongly dependent on the mixing technique used. Under conditions of excess chromium(II), more than the stoichiometric amount of Cr(III) products were isolated, indicating that some degree of substitution was occurring. Blank experiments established that the products obtained could have arisen from an outer-sphere reaction followed by capture of the free ligands by Cr(II). A synergistic effect between the acac and bipy ligands appears to be operating. Two reactions were observed on the stopped-flow instrument. Both are ascribed to ring-closure of bipy on Cr(III). The reaction to give [Cr(bipy)(OH2)4]3+ was characterised by k25 °C = 5.06 × 10-1 s-1 with activation parameters ΔH≠ = 9.9 ± 0.3 kcal/mol, and ΔS≠ = -26.8 ± 0.9 e.u. The second reaction was ascribed to the formation of [Cr(bipy)(ptdn)(OH2)2]2+ from [Cr(bipy)(ptdn)(OH2)3]2+ and had k25 °C = 5.83 × 10-2 s-2; ΔH≠ = 4.93 ± 0.06 kcal/mol, ΔS≠ = 47.6 ± 0.2 e.u. The final reaction was attributed to breakdown of the Cr(III) products. It occured very slowly with k25 °C = 4.6 × 10-6 s-1 (all values quoted at μ = 0.10 M (LiClO4) and [H+] = 0.030 M).

AB - The Cr(II) reduction of [Co(bipy)2(ptdn)2+ (bipy = 2,2t́-bipyridine and ptdn = pentane-2,4-dione) proceeded too rapidly to be detected by the stopped-flow method and a lower limit of 3 × 104 M-1 s-1 (μ = 0.10 M (LiClO4)) and [H+] = 0.030 M was calculated for this reaction. Product studies established that both [Cr(bipy)(OH2)4]3+ and [Cr(bipy)(ptdn)(OH2)2]2+ were formed. The ration of products obtained was strongly dependent on the mixing technique used. Under conditions of excess chromium(II), more than the stoichiometric amount of Cr(III) products were isolated, indicating that some degree of substitution was occurring. Blank experiments established that the products obtained could have arisen from an outer-sphere reaction followed by capture of the free ligands by Cr(II). A synergistic effect between the acac and bipy ligands appears to be operating. Two reactions were observed on the stopped-flow instrument. Both are ascribed to ring-closure of bipy on Cr(III). The reaction to give [Cr(bipy)(OH2)4]3+ was characterised by k25 °C = 5.06 × 10-1 s-1 with activation parameters ΔH≠ = 9.9 ± 0.3 kcal/mol, and ΔS≠ = -26.8 ± 0.9 e.u. The second reaction was ascribed to the formation of [Cr(bipy)(ptdn)(OH2)2]2+ from [Cr(bipy)(ptdn)(OH2)3]2+ and had k25 °C = 5.83 × 10-2 s-2; ΔH≠ = 4.93 ± 0.06 kcal/mol, ΔS≠ = 47.6 ± 0.2 e.u. The final reaction was attributed to breakdown of the Cr(III) products. It occured very slowly with k25 °C = 4.6 × 10-6 s-1 (all values quoted at μ = 0.10 M (LiClO4) and [H+] = 0.030 M).

UR - http://www.scopus.com/inward/record.url?scp=26344454104&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=26344454104&partnerID=8YFLogxK

U2 - 10.1016/S0020-1693(00)87964-5

DO - 10.1016/S0020-1693(00)87964-5

M3 - Article

AN - SCOPUS:26344454104

VL - 99

SP - 169

EP - 176

JO - Inorganica Chimica Acta

JF - Inorganica Chimica Acta

SN - 0020-1693

IS - 2

ER -