Hydrogenation by cyanocobaltate. Part 5. Pseudo-contact shift by ion association between organocyanocobaltates(III) and tris(ethylenediamine)cobalt(II) ion in the catalytic system of tricyano(ethylenediamine)cobaltate(II)

Abstract

The pseudo-contact shift observed with alkylpentacyanocobaltates(III) and tetracyano(η-methylallyl)cobaltates(III) has been explained by the formation of ion pairs such as [Co(en)₃]²⁺,[Co(CN)₅R]^3–(en = ethylenediamine; R = CH₂CH:CMe₂, CH₂Ph, or CH₂C₆H₄Me-o, -m, or -p). The equilibrium constant for the reaction [Co(en)₃]²⁺+[Co(CN)₅R]^3–⇌[Co(en)₃]²⁺,[Co(CN)₅R]^3– has been estimated to be 0.67 ± 0.02 dm³ mol^–1 when R = CH₂C₆H₄Me-o. The observed pseudo-contact shifts are qualitatively explained by calculations on simple models of the ion pairs, in which [Co(en)₃]²⁺ is located on the principal axis of the cyano-complexes at the opposite side of the organic ligands and the distance between the two cobalt atoms is ca. 5.5–6 Å. Steric configurations of the organic ligands are discussed.