σ-Bonded organotransition-metal ions. Part IX. Strong hyperconjugative and inductive electron donation by some alkyl-manganese, -molybdenum, -tungsten, and -iron carbonyl groups in substituted pyridinium ions

Abstract

Several pyridylmethylmetal compounds and their conjugate acids have been prepared. The acid dissociation constants of the pyridinium ions have been measured spectrophotometrically. The results show that the groups CH₂Mn(CO)₅, CH₂Mo(CO)₃(π-C₅H₅), CH₂W(CO)₃(π-C₅H₅), and CH₂Fe(CO)₂(π-C₅H₅) are extremely strong electron-donating substituents whether in the meta- or para-positions. Approximate σ-values are assigned to these groups and the effects are ascribed to a combination of inductive and hyperconjugative electron donation.