Physical organic chemistry of transition metal carbene complexes. 10. Opposing effects of alpha-alkyl groups on thermodynamic and kinetic acidities of (CO2)5Cr=C(OMe)CHR'R'-type fischer carbene complexes in aqueous acentonitrile. Analogy to the nitroalkane anomaly

The pKa values for transition metal carbene complexes (CO2)5Cr=C(OMe)CH2CH3 (3-Et), (CO)5(Cr=C(OMe)CH2CH2CH3) (3-Bu) and (CO)5CR=C(OMe)CH(CH3)2 (3-Pr) in aqueous acetonitrile are reported as 12.62, 12.84 and 12.27 respectively. They are all more acidic than (CO5)Cr=C(OMe)CH3 (3-Me). In contrast to their thermodynamic acidities, 3-Et, E-Bu and 3-PR all exhibit greater rate constants than 3-Me. Other research observations are made. Kinetic reaction parameters are given together with data from spectrophotometer analysis.