Steric, electronic and solvation effects in the co-ordination of amines by the [Fe(CN)5(OH2)]3- ion

Equilibrium constants K for the substitution of co-ordinated H2O in [FeII(CN)5(OH2)]3- by sterically hindered amines (both α and β branching, viz. NH2Me–NMe3 and NH2Me–NH2But) and by aniline have been determined in aqueous solution with I = 0.1 mol dm-3 (NaClO4) at 25 °C by UV/VIS spectrophotometry. Comparison with published data on the rates of Fe–N bond fission and their further correlation with the energetics of protonation of the free amines shows that the observed variation in log K for aliphatic amines reflects ‘solvation’ as well as more typically steric effects. Aniline differs from the aliphatic amines both in its smaller crystal-field splitting (shared by other aromatic amines) and a low value of log K comparable to that of NMe3.