COORDINATION OF AMINES TO [FE(CN)(5)(OH2)](3-)

Equilibrium constants K for the substitution of co-ordinated H2O in th e complex [Fe(CN)(5)(OH2)](3-) by amines have been determined in 0.1 m ol dm(-3) aqueous acetonitrile solution with I = 0.1 mol dm(-3) (NaClO 4) at 25 degrees C by UV/VIS spectrophotometry. The species previously assigned to the NH2OH complex is shown to be the product of some oxid ative side-reaction. In the absence of complicating factors, values of log K obey the linear free-energy relation log K = a . pK + b with a = 0.05 and b = 3.75 for over 11 pK units from NH2NH3+ (pK -0.9) to NH( 2)Me (pK 10.6), i.e. log K is virtually independent of pK. It is sugge sted that log K may become independent of pK where the increase in int rinsic 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 wit h complexes of Fe-III and Co-III, a decrease in log K (below that expe cted from basicity alone) is observed for glycine (attributed to coulo mbic repulsion between the negative charges on the ligand and complex) , NH3 and NH2OH (both attributed to repulsion from the additional stro ngly held solvent molecules).