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 dm^3–(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 Fe^III and Co^III, 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).