Substitution and Isomerization of Nitrite Ion at a Cobalt(III) Centre Having a Coordinated Pentadentate Ligand with Two Nitrogen and Three Sulfur Donor Atoms

Abstract

The influence of placing thioether linkages trans to a site of nitrito substitution and spontaneous nitrito-tonitro isomerization is reported for the [CoQS(H2O)]3+ cation where QS is 1,11-diamino-3,6,9-trithiaundecane. Preparation and characterization is described for the aqua and nitrito complexes. Rate data for the substitution process is presented at 17.7, 25.0 and 35.0°C. It is consistent with the mechanism first proposed by Basolo and Pearson in which N2O3 is the nitrosation agent. [CoQS(H2O)]3+ is three hundred times more reactive than [Co(NH3)5H2O]3+ under identical conditions. Isomerization is dramatically slower than the conversion of [CoQS(H2O)3+ to [CoQS(ONO)]2+. The isomerization process was studied at 5 wavelengths, 3 temperatures and various conditions of acid and nitrite ion at an ionic strength of 0.11–0.60 M. Studies at 25°C give kisom = 1.21 ± 0.12 × 10−4 sec −1. Similar determinations at 17.7 and 35.0°C give kisom = 3.84 ± 0.65 × 10−5 sec−1 and 3.59 ± 0.13 × 10−4 sec−1 respectively. The thermodynamic activation parameters ΔH≠, ΔG≠, and ΔS≠ obtained from an Eyring plot gives ΔH≠ = 111.3 kJ/mol, ΔS≠ = + 53 J/molK and ΔG≠ = 95.4 kJ/mol. These results are discussed in the context of present knowledge and experience with other cobalt(III) ligand systems.