The fundamental flaw of the HSAB principle is revealed by a complete speciation analysis of the [PtCl6−nBrn]2− (n = 0–6) system

Abstract

Bjerrum's model of step-wise ligand exchange is extended to compute a complete speciation diagram for the [PtCl_6−nBr_n]²⁻ (n = 0–6) system including all 17 equilibrium constants concerning the Pt^IV chlorido–bromido exchange reaction network (HERN). In contrast to what the hard soft acid base (HSAB) principle “predicts”, the thermodynamic driving force for the replacement of chloride by bromide in an aqueous matrix, for each individual ligand exchange reaction present in the Pt^IV HERN, is due to the difference in halide hydration energy and not bonding interactions present in the acid–base complex. A generalized thermodynamic test calculation was developed to illustrate that the HSAB classified class (b) metal cations Ag⁺, Au⁺, Au³⁺, Rh³⁺, Cd²⁺, Pt²⁺, Pt⁴⁺, Fe³⁺, Cd²⁺, Sn²⁺ and Zn²⁺ all form thermodynamically stable halido complexes in the order F⁻ ≫ Cl⁻ > Br⁻ > I⁻ irrespective of the sample matrix. The bonding interactions in the acid–base complex, e.g. ionic–covalent σ-bonding, Π-bonding and electron correlation effects, play no actual role in the classification of these metal cations using the HSAB principle. Instead, it turns out that the hydration/solvation energy of halides is the reason why metal cations are categorized into two classes using the HSAB principle which highlights the fundamental flaw of the HSAB principle.