Mechanistic studies on the bridge cleavage reaction of the dinuclear ethylenedibiguanide complexes of ruthenium-, cobalt-, chromium- and rhodium(III) in acidic medium

Acid-catalysed bridge cleavage reactions of four metal-ethylene dibiguanide (EDG) (metal = RuIII, CoIII, CrIII, RhIII complexes have been investigated in perchloric acid and for RuIII complex in nitric and paratoluenesulphonic acids also. The CoIII complex reacts both via acid-dependent and acid-free paths, while other complexes by acid-dependent path only. Rate law for the RuIII and CrIII complexes is kobs = k1 (H+), that of the CoIII complex is kobs = k2 (H+) + k3 and for the RhIII complex is kobs = k4K/(K + H+), where K is the equilibrium constant for protonation. The mechanism involves protonation equilibrium of respective complex, followed by a rate-determining bridge cleavage. Activation parameters have also been calculated. Order of reactivity of complexes is : CrIII > CoIII > RuIII > RhIII. (author)