Patterns in Lewis acid-base interactions in aqueous solution

Abstract

The observable patterns in free energies of complex-formation in aqueous solution have lead to many ▪ ▪ classifications (A and B, ‘hard’ and soft) of Lewis acids and bases. These can be illustrated with LFER (linear free energy relationship) diagrams [1]. Figure 1 is consistent with Pearson's ideas on hard and soft acids and bases (which are interpreted as representing ionic and covalent bonding) if Ag(I) is softer than Hg(II). figure 2 displays the same pattern as Fig. 1, but is inverted, indicating that Bi(III) is harder than Hg(II). These patterns, which are similar for all metal ions, may be reproduced using eqn. 1, which is essentially that due to Drago and coworkers [2]. ▪ E and C are the tendency of the Lewis acid A and base B to undergo ionic and covalent bonding. Eqn. 1 is not able to correlate formation constants for complexes of ligand having large donor atoms (S, P, As, Cl, Br, I)with smaller Lewis acids (Cu(II), Co(III), the proton), but works well for small ligands (F −, OH −, NH 3) with all Lewis acids. The deviation from the prediction of eqn. 1 are interpreted as being due to ( i) steric hindrance to solvation of the Lewis acid by large donor atoms, and ( ii) specific solvation effects, which operate through the increased softness induced in the Lewis acid by attachment of very soft donor atoms. Mechanism ( i) is supported by evidence from the correlations in the NMR and infra-red involving the structural trans effect, while mechanism ( ii) is supported by results on proton-basicities of organic bases in the gas-phase. Eqn. 1 is extended to give eqn. 2, which correctly predicts the formation constants of 266 different ▪ complexes involving 31 different Lewis acids (including the proton) with 16 different unidentate bases. The D parameters are identified with desolvation effects arising form mechanism ( i) and ( ii) above.