The Effect of Pressure on the Anation of Diaquabis(ethane-1,2-diamine)cobalt(III) by Selenate and Sulfate, and on the Isomerization of the trans-Aquabis(ethane-1,2-diamine)(hydrogenselenito)cobalt(III) Ion

Abstract

The reversible nucleophilic substitution of cis-[Co(en)₂(OH₂)₂]³⁺ (en = ethane-1,2-diamine) by sulfate and selenate at pH 2.0 was studied at elevated pressure, and activation volumes (ΔV‡) for anation and solvolysis were determined. The ΔV‡ values for anation by sulfate and selenate were +8.3±0.5 and +8.5±0.4 cm³ mol^-1 respectively, whereas solvolysis of sulfato and selenato complexes cis -[Co(en)₂(OH₂)(XO₄)]⁺ gave ΔV‡ values of -2.2±0.4 and -3.3±0.7 cm³ mol^-1 respectively. The trans → cis [Co(en)₂(OH₂)(OSeO₂H)]²⁺ isomerization studied at pH 1.0 and elevated pressure yielded ΔV‡ + 7.2±0.4 cm³ mol^-1. All reactions can be interpreted in terms of dissociative mechanisms.