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Formation And Kinetics Of Dissociation Of Some Pentaaquotrihalomethyl Chromium(Iii) Ions.

Author

Sher Khan Malik, University of the Pacific

Date of Award

1973

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

First Advisor

Larry O. Spreer

First Committee Member

Emerson G. Cobb

Second Committee Member

Carl E. Wulfman

Third Committee Member

Herschel Frye

Fourth Committee Member

Donald K. Wedegaertner

Fifth Committee Member

Richard P. Dodge

Abstract

The present.study was an investigation of the formation and kinetic stability of store complexes contain.ir)ing a chromium-carbon bond in an aqueous medium. An orange-brown pentaaquotrifluoromethylchromium(III) ion was obtained by the reduction of trifluororrethyl iodide with Cr(II) and is the most inert of any organochrotnium(III) complex known. The initial aquation rates of trifluoromethylchromium(III) ion were described by the differential rate law -dln [(H₂O)₅CrCF₃²⁺]/dt = k_{o + k1(H⁺)}

The products of the initial reaction were hexaaquochromium (III), carbon monoxide and HF. The values of k_o and k₁ at 45° were 5.21 x 10^-1 and 9.38 x 10M^-1sec^-1 respectively. The activation parameters for the acid independent and acid dependent pathways were: ΔH⁺ = 19.7 kcal/more, ΔS⁺ = -27.4 cal-deg^-1mole^-1; and ΔH^ǂ₁ = 23.4 kcal/mole and ΔS^ǂ₁ = -12.7 cal-deg^-1mole^-1.

The acceleration in the aquation with time was observed indicating autocatalysis by a product of the reaction. An intermediate, (Cr( OH₂)₄FCrCF₃¹⁺ ), was separated from the aquation reaction solution; this same ion was also formed immediately when fluoride ion was added to the (H₂)₅CrCF₃²⁺ ion in 0.05M perchloric acid. The added fluoride ion also increased the rate of aquation. Rapid formation of (Cr(OH₂)₄FCrCF₃¹⁺) ion was attributed to the strong stabilizing effect of the trifluoromethyl ligand on water molecules in the inner coordination sphere of chromium( III).

A solvent-assisted mechanism for the aquation of pentaaquotrifluoromethyl-chromium(III) ion was proposed in which an activated complex was formed which dissociated in a concerted manner to give the products. Also, a reaction sequence was proposed for the complete aquation reaction which was consistent with the rate laws, the effect of added fluoride ion and the activation parameters.

Pink organochromium(III) species of 2+ charge was isolated in the reduction of carbon tetrachloride and carbon tetrabromide with chromium( II). he pink species have ·chromium:carbon:halide ratios of 1:1:1 and their uv-visible spectra showed unusually high absorbance values in the 500 nm region. The pink organochromium(III) reacted very rapidly with oxygen. The products of dissociation were carbon monoxide, formic acid, formaldehyde, bexaaquochromium(III), chromium( II) halide ions. The kinetics of aquation were studied, and initial rate coefficients and activation parameters were obtained. The identity dt these organochromium(III) species was not established but most possible structures were proposed.

Pages

134

Recommended Citation

Malik, Sher Khan. (1973). Formation And Kinetics Of Dissociation Of Some Pentaaquotrihalomethyl Chromium(Iii) Ions.. University of the Pacific, Dissertation. https://scholarlycommons.pacific.edu/uop_etds/3049