The Hard-Soft Acid-Base (HSAB) principle is a conceptual tool in inorganic chemistry that describes the tendency of certain Lewis acids and bases to form more stable interactions based on their "hard"...The Hard-Soft Acid-Base (HSAB) principle is a conceptual tool in inorganic chemistry that describes the tendency of certain Lewis acids and bases to form more stable interactions based on their "hard" or "soft" characteristics.