Co-ordination of amines to [Fe(CN)5(OH2)]3?

Abstract

Equilibrium constants K for the substitution of co-ordinated H2O in the complex [Fe(CN)5(OH2)]3– by amines have been determined in 0.1 mol dm–3 aqueous acetonitrile solution with I= 0.1 mol dm3–(NaClO4) at 25 °C by UV/VIS spectrophotometry. The species previously assigned to the NH2OH complex is shown to be the product of some oxidative side-reaction. In the absence of complicating factors, values of log K obey the linear free-energy relation log K=a·p K+b with a= 0.05 and b= 3.75 for over 11 pK units from NH2NH3+(pK–0.9) to NH2Me (pK 10.6), i.e. log K is virtually independent of pK. It is suggested that log K may become independent of pK where the increase in intrinsic metal–ligand bond energy with pK is balanced by the increase in the overall loss in solvation (including hydrogen-bonding) energy on co-ordination with pK. In contrast to the pattern previously found with complexes of FeIII and CoIII, a decrease in log K(below that expected from basicity alone) is observed for glycine (attributed to coulombic repulsion between the negative charges on the ligand and Complex), NH3 and NH2OH (both attributed to repulsion from the additional strongly held solvent molecules).