Volumes of activation for dissociative (D) mechanisms: the substitution of pentacyanoferrate(II) complexes by various nucleophiles

Abstract

The dissociative step of a dissociative, or D, substitution mechanism exhibits a volume of activation, ΔV₁^‡, which is independent of the nature of the incoming nucleophile. For the reaction of [Fe(CN)₅(3,5Me₂-py)]^3–(3,5Me₂-py = 3,5-dimethylpyridine) with [CN]^–, ΔV₁^‡= 20.5 ± 0.8 cm³ mol^–1; with pyrazine, ΔV₁^‡= 21.2 ± 1.0 cm³ mol^–1; and with imidazole, ΔV₁^‡= 20.3 ± 1.0 cm³ mol^–1. For the reaction of [Fe(CN)₅(3CN-py)]^3–(3CN-py = 3-cyano-pyridine) with [CN]^–, ΔV₁^‡= 20.6 ± 0.5 cm³ mol^–1. These volumes of activation are shown to correspond to stretching of the Fe–N bond in the activated complex from 65 to 88% of the distance corresponding to complete dissociation of the leaving group from the [Fe(CN)₅]^3– moiety.