HSABrammeverket

HSABrammeverket, or the HSAB framework, is a heuristic used in inorganic, organometallic, and coordination chemistry to rationalize and predict the outcome of acid–base interactions. It is based on the hard and soft acids and bases (HSAB) theory proposed by Ralph Pearson in 1963. The framework classifies chemical species as hard or soft acids and bases: hard species are small, highly charged, and poorly polarizable; soft species are larger, with high polarizability and greater covalent character. Borderline species exhibit intermediate properties.

Interactions are favored when the hardness or softness of interacting partners matches: hard–hard and soft–soft combinations tend to form more stable adducts, while mismatched pairs are disfavored or slower to form. This framework helps explain trends in complex formation, ligand binding in transition metal complexes, and catalytic selectivity. It is often used as a first-pass guide in selecting ligands and metals for a desired reactivity.

Limitations: the HSAB framework is qualitative and ideally complemented by more detailed electronic structure calculations. Solvent effects, covalent bonding, and kinetic factors can override simple hard/soft considerations, and many species are borderline between categories, reducing predictive power.

Extensions and modern use: researchers quantify hardness and softness and use computational methods to place species on a continuum rather than discrete categories. The HSABrammeverket remains a foundational heuristic for understanding reactivity patterns in inorganic chemistry and related fields.

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