Lewis acid-base concept – a unifying principle applicable to soil system, geochemistry and biology

Author: Raina Niskanen

Received 18 May 2006, accepted 12 August 2006.

Abstract

The concept of hard and soft Lewis acids and bases was reviewed as a unifying principle in the chemistry of soil system, in the geological distribution of elements and in living systems. The common aquatic character of natural environments with the important role of hard-hard interactions and the factors determining the bond strength were discussed. Further evaluations included the complexation tendency of hard ions in water solution which is parallel with the standard entropy of ions. In the soil system, these principles manifest as increased sorption ability of oxyacid ligands with increasing pKa of the acid, while the complexation of hydrated metal cations is enhanced with decreasing pK of hydrolysis. From mineralogical and metallurgical points of view the softness is expressed as poor hydration of inert noble metals. In biological systems, coordination environments for metals range from very hard oxygen donors to very soft sulphur donors.