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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 14, Number 6
BKCSDE 14(6)
June 20, 1993 

Kinetics and Stereochemistry of CO Substitution Reactions of Half-Open Chromocene Carbonyls(Ⅱ) : Reactions of Cp(η5-2,4-Me2C5H5)CrCO and Phosphines
Jong-Jae Chung*, Byung-Gill Roh
The CO substitution reactions of the complex, Cp(S-2,4-Me2C5H5)CrCo with PR3(PR3=PMePh2, P(OCH3)3, PMe2Ph) were investigated spectrophotometrically at various temperatures. From the reaction rates, it was suggested that the CO substitution reaction took place by first-order (dissociative) pathway. Activation parameters in decaline were △H≠= 22.0 kcal·mol-1, △S≠= -3.8 cal·mol-1·K-1. Unusually low value of △S≠ suggests an η5-S→η5-U conversion of the pentadienyl ligand. This suggestion was confirmed by the Extended-Huckel molecular orbital (EHMO) calculations, which revealed that the total energy of Cp(S-2,4-Me2C5H5)CrCO is about 0.42 kcal/mol more lower than that of Cp(U-2,4-Me2C5H5)CrCO and the energy of [Cp(U-2,4-Me2C5H5)Cr…CO]≠ transition state is about 2.43 kcal/mol lower than that of [Cp(S-2,4-Me2C5H5)Cr…CO]≠ transition state.
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