Kinetics and mechanism of dissociation of some para-substituted phenylbiguanide complexes of nickel(II) in aqueous hydrochloric acid

Abstract

The rates of dissociation in aqueous hydrochloric acid medium of the bis complexes of nickel(II) with N¹(4-methoxyphenyl) biguanide and N¹-(4-sulphophenyl)biguanide have been investigated by stopped-flow spectrophotometry and the results compared with those of the biguanide and N¹-phenylbiguanide complexes reported earlier. The complete dissociation of the complexes to the Ni(H₂O)<img src='http://www.niscair.res.in/jinfo/equation27.gif' border=0> and free ligand (protonated) occurs in two consecutive steps, the first being relatively faster than the second; each step leading to loss of one ligand. For each step the rate is first order in acid concentration. The results are consistent with a solvent-assisted dissociative process involving the protonated complex; the rate varies due to the electronic effect of the substituent in the phenyl ring. For each of the steps isokinetic trend has been observed, and the biguanide complex also fits into the trend. The relative rates are moreor less enthalpy controlled.