Structural and mechanistic studies of co-ordination compounds. Part 19. Acid hydrolysis of some trans-tetra-aminedichlororuthenium(III) cations

Abstract

The preparation and characterization of trans-[RuCl₂{(R,S)-3,7NH-nd}]⁺(3,7NH-nd = 3,7-diazanonane-1,9-diamine) are described. A study of the acid-hydrolysis and isotopic chloride-exchange reactions of complexes of the type trans-[RuCl₂L]⁺ has indicated that the reactions are stereoretentive and that the first-order rate constants decrease with increasing chelation of L: (NH₃)₄≈(en)₂>(R,S)-3,7NH-nd > cyclam (en = ethylenediamine, cyclam = 1,4,8,11-terra-azacyclotetradecane), with a corresponding increase in the enthalpies of activation. The entropies of activation have negative values. A dissociative mechanism involving square-pyramidal intermediates has been suggested. The relation between the acid hydrolysis of both the cis and trans series of tetra-amine-dichlororuthenium(III) cations with that of corresponding cobalt(III), chromium(III), and rhodium(III) complexes is discussed. The importance of the σ-trans effect of nitrogen donors, the nephelauxetic effect of amine ligands, and solvation effects have been invoked to explain the chelation and cis versus trans effects of amine ligands on the acid hydrolysis of these complexes.