Kinetics and Mechanisms of Metal Carbonyl Radical Reactions

Wegman, Richard William

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Description

Title

Kinetics and Mechanisms of Metal Carbonyl Radical Reactions

Author(s)

Wegman, Richard William

Issue Date

1981

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Chemistry, Inorganic

Language

eng

Abstract

• Dinuclear metal carbonyl compounds, M(,2)(CO)(,2m)L(,2n), generally possess an intense UV absorption corresponding to a (sigma) (--->) (sigma)* transition of an electron in the metal-metal bond. Irradiation into this absorption results in formation of the (.)M(CO)(,m)L(,n) radicals with high quantum efficiencies.
• Flash photolysis of CO-saturated hexane solutions of M(,2)(CO)(,10) (M = Mn, Re) and Co(,2)(CO)(,6)L(,2) (L = CO, P(n-Bu)(,3)) results in biomolecular recombination of the (.)M(CO)(,m)L(,n) radicals with rate constants on the order of 10('8)-10('9)M('-1)s('-1). Flash photolysis of CO-free hexane solutions of M(,2)(CO)(,10) results in formation of a short-lived intermediate. The transient is not observed when CO is present, suggesting that the species is one or more coordinatively unsaturated manganese or rehenium dinuclear metal carbonyl compounds. Presumably, thermal dissociation of CO from (.)M(CO)(,5) results in formation of M(CO)(,4) and subsequent reaction of M(CO)(,4) with (.)Mn(CO)(,5) and M(CO)(,4) leads to transient formation.
• The flash photolysis results support the notion that thermal CO dissociation from M(CO)(,m)L(,n) is a facile process. (.)M(CO)(,(m-1))L(,n) is a 15-electron, coordinatively unsaturated species. In some instances, metal hydrides, HML(,x), oxidatively add to (.)M(CO)(,(m-1))L(,n) resulting in formation of (.)M(CO)(,m-1)L(,n)(H)(ML(,x)). This species undergoes a hydrogen atom transfer reaction with HML(,x) resulting in formation of (H)(,2)M(CO)(,m-1)L(,n)(ML(,x)). Reductive elimination of H(,2) followed by reaction with CO leads to formation of L(,x)M-M(CO)(,n)L(,m).
• The mechanism described above is quite general. For example, the reaction of (.)Co(CO)(,3)L and (.)M(CO)(,3)Cp (Cp = (eta)('5)-C(,5)H(,5), M = Mo, W) with HSnBu(,3) results in information of L(CO)(,3)CoSnBu(,3) and Cp(CO)(,3)MoSnBu(,3). The initial step in each case is CO loss from (.)M(CO)(,m)L(,n). The reaction of (.)Co(CO)(,3)L with HMn(CO)(,5) results in formation of L(CO)(,3)CoMn(CO)(,5). However, reaction of (.)Mo(CO)(,3)Cp with HMn(CO)(,5) proceeds via a hydrogen atom transfer process. The photochemical and thermal decomposition reactions of HCo(CO)(,4) are best accounted for by a mechanism involving cobalt-centered carbonyl radicals. The postulated mechanism is analogous to that suggested for the reaction of (.)M(CO)(,m)L(,n) with HML(,x).
• Hydrides with large M-H bond energy, i.e., HSnBu(,3) ((DBLTURN) 70 kcal/mole) are likely to react with (.)M(CO)(,m)L(,n) via an oxidative addition process. That is, CO dissociation from (.)M(CO)(,m)L(,n) competes favorably with a hydrogen atom transfer process. When the M-H bond energy is low, i.e., HMn(CO)(,5) ((DBLTURN) 45 kcal/mole) the reaction mechanism could be determined by the abstracting property of (.)M(CO)(,m)L(,n).
• The reaction of HCo(CO)(,3)L with L(L = P(n-Bu)(,3)) results in formation of HCo(CO)(,2)L(,2). An inductive period, typically 10-50 seconds, is followed by rapid conversion (t(, 1/2) = 15-20 seconds) of HCo(CO)(,3)L into HCo(CO)(,2)L(,2). A free radical chain mechanism is postulated. The key step is loss of CO from (.)Co(CO)(,3)L followed by reaction of (.)Co(CO)(,2)L with L.
• The reaction of HMo(CO)(,3)((eta)('5)-C(,5)H(,5)) with HSnBu(,3) results in formation of Bu(,3)SnMo(CO)(,3)((eta)('5)-C(,5)H(,5)). Interestingly, the reaction is not inhibited by added CO. At this juncture an (eta)('5) () (eta)('1) equilibrium of the C(,5)H(,5) ligand followed by reaction of HSnBu(,3) and reductive elimination of H(,2) seems the most reasonable process leading to formation of Bu(,3)SnMo(CO)(,3)((eta)('5)-C(,5)H(,5)).

Graduation Semester

1981

Type of Resource

text

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