Influence of cationic polyelectrolytes on the kinetics and equilibria in pentacyanoiron(II) systems

Abstract

The addition of a cationic polymer [poly(vinylbenzyltrimethylammonium), PVBA] increases the rate of release of aliphatic amines L from [Fe(CN)₅L]^3– by a factor that increases with increasing ratio R=[PVBA]/[Fe], and reaches a maximum of 2–3 at R= 8–10. For aromatic amine ligands, no rate enhancement is observed. The rate of the reverse formation reaction of [Fe(CN)₅L]^(n–3) from [Fe(CN)₅(H₂O)]^3– and Lⁿ is affected only for n≠ 0, there being rate enhancement for anionic ligands and a rate decrease for cationic ligands. The influence on the rate of release of aliphatic amines is interpreted in terms of interactions between PVBA chains and the releasing ligand, whilst the rate changes in the reverse reaction are typical of the so-called ‘polyelectrolyte effect’ in bimolecular reactions. The dependence on PVBA molality of the rate of complex second-order reactions involving ion-pair formation as a pre-equilibrium is presented. By combining the effect on forward and reverse reaction, the influence of PVBA is shown to correspond to a linear free-energy relationship, with slope unity; this is in agreement with a mechanism involving dissociative activation. The lack of influence of PVBA on the rate of release of aromatic amines is probably due to compensating effects, as spectral changes suggest the possible operation of a parallel decrease in the rate constant. PVBA also affects the equilibrium between [Fe(CN)₅NO]^2– and [Fe(CN)₅NO₂]^4– in alkaline media; the pK value is shifted by up to two units (for R= 12); further effects are observed related to the existence of other equilibria and/or slow irreversible processes that are also affected by the polycation.