Stability constants of SnCl 2-pyridine N-oxide complexes in acetonitrile

Abstract

Stability constants have been determined for eight methyl or chloro-substituted pyridine N-oxide adducts of SnCl 2 in acetonitrile solution. The complexes have a 1:1 stoichiometry in the concentration ranges studied. An IR spectrophotometric method was utilized to determine concentrations of the equilibrium species from the NO stretching band of the free ligand and from mass balance relationships. The equilibrium concentrations are such that the NO stretching band of the complexed ligand also appears in most of the spectra, shifted to lower frequencies by 50–75 cm −1. The stability constants were determined with an estimated error of 7–10%. Values of these constants for the formation of the complex SnCl 2·L where L is the N-oxide are: pyridine N-oxide, 22; 2-picoline N-oxide, 51; 3-picoline N-oxide, 243; 4-picoline N-oxide, 78; 2,6-lutidine N-oxide, 28; 2-chloropyridine N-oxide, 98; 3-chloropyridine N-oxide, 29; and 4-chloropyridine N-oxide, 146. The values of the stability constants of the complexes are rationalized in terms of electronic and steric effects of the substituents on the pyridine N-oxide ring.