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Reactions of [Aryloxy(phenyl)carbene]pentacarbonylchromium(0) Complexes with Thiolate Ions. Decreasing Reactivity with Increasing Basicity of the Nucleophile
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A kinetic study of the reactions of thio
late ions with three Fis
cher-type [ary
loxy(pheny
l)carbene]pentacarbony
l chromium(0) comp
lexes in 50% MeCN-50% water (v/v) is reported. Brnsted p
lots ofthe second-order rate constants are biphasic with an initia
l steep rise for weak
ly basic thio
late ions (
nuc 1.0) fo
llowed by a s
light
ly descending
leg with a negative s
lope (
nuc -0.2) for strong
ly basicthio
late ions. This indicates a
change from rate-
limiting
leaving group departure at
low p to rate-
limiting nuc
leophi
lic atta
chment at high p. The negative
nuc va
lues resu
lt from a combination ofminima
l progress of C-S bond formation at the transition state and the requirement for partia
l deso
lvationof the nuc
leophi
le before it enters the transition state. Possib
le factors that may affect the degree of bondformation in reactions of Fis
cher carbene comp
lexes as we
ll as reactions of other unsaturated e
lectrophi
leswith thio
late ions are discussed.