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|Title:||Kinetics and Mechanisms of Substitution Reactions of Dicobalt Octacarbonyl|
|Author(s):||Forbus, Nancy Page|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||Reaction of Co(,2)(CO)(,8) with bases of low nucleophilicity, e.g. P(CH(,2)CH(,2)CN)(,3) or P(O-i-Pr)(,3), or large steric requirement, e.g. P(t-Bu)(,3), results in formation of substituted dinuclear species of the form Co(,2)(CO)(,7)L and Co(,2)(CO)(,6)L(,2). These reactions exhibit first order dependence on concentration of carbonyl, no dependence on concentration of base, and inhibition of rate of reaction by added CO.
With very nucleophilic bases of small steric requirement, e.g. P(n-Bu)(,3) or P(i-Pr)(,3), ionic products of the form Co(CO)(,3)L(,2)('+)Co(CO)(,4)('-) are observed. These reactions exhibit non-integral order dependence on concentration of carbonyl and base. Observed order in carbonyl is temperature-dependent; order in carbonyl decreases as temperature increases.
These results suggest two competing mechanisms. One of these involves a rate-determining CO dissociation. The coordinatively unsaturated carbonyl species reacts with any nucleophile in solution to give substituted dinuclear products. A more extraordinary pathway for substitution involves an associative attack at Co(,2)(CO)(,8), forming an intermediate Co(,2)(CO)(,8)L. This intermediate may undergo metal-metal bond rupture with CO expulsion to form 17-electron metal carbonyl radicals. Through an outer-sphere electron transfer reaction, the Co(CO)(,3)L('.) species thus formed initiates a radical chain process leading to formation of the observed ionic product. A computer modeling study of this radical mechanism was able to simulate the observed kinetics very closely.
The nucleophilicity and steric requirement of the base determine which of two possible pathways will be followed and the rate of the reaction. In order for a rapid radical chain reaction to be observed, the base must have a high nucleophilicity and a small steric requirement. For bases of intermediate steric requirement and high nucleophilicity, reaction may still occur by the radical chain mechanism, but at a rate much slower than is observed with the smaller nucleophilic base. If the steric requirement of the base becomes too great, or the base is of fairly low nucleophilicity, the radical chain mechanism cannot occur and the CO dissociative mechanism will be observed.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1981.
|Date Available in IDEALS:||2014-12-13|