Co-ordination of nitriles to [Fe(CN)5(OH2)]3– and use of the acetonitrile complex for the determination of equilibrium constants

Abstract

Solid Na₃[Fe(CN)₅(NH₃)], which hydrolyses rapidly in solution to the unstable complex [Fe(CN)₅(OH₂)]^3–1, when dissolved in aqueous acetonitrile gave a solution of the much more stable complex [Fe(CN)₅(MeCN)]^3– with log K 2.6 (25 °C, I= 0.1 mol dm^–3). It was shown that the use of solutions of the acetonitrile complex instead of 1 provides a general method for determining equilibrium constants for ligand substitution (directly of MeCN, hence indirectly of H₂O in 1) under equilibrium conditions and over a wider range of pH from 3 to 13. Values of log K have also been determined for other nitriles such as PhCN (3.5), NCCH₂NH₃⁺(3.3), NCCH₂CO₂^–(2.5) and NCNH₂(2.3), also for NH₃(4.2), NH₂CH₂CN (3.7) and pyridine (3.7). The data suggest that, in the absence of complicating factors, the values of log K for nitriles show little dependence on their basicity. Comparison of present and published data on the wavelength of the first d–d transition in these and analogous complexes shows that the position of a nitrogen-containing base in the spectrochemical series depends primarily on the state of hybridisation of the donor N atom and increase in the order H₂O (443) < sp³ N (all 400 ± 4) < sp² N (all 383 ± 3) < sp N (all 375 ± 2) < CN^–(325 nm).