6.6 HSAB Theory

Positive and negative ions attract each other, so a given ion would be expected to seek out and bond with the strongest available opposite charge ion. Indeed, reactions between salts do generally favor pairing the ions with the greatest extremes of electronegativity, leaving the milder ions to combine with each other. But this does not explain the behavior of metals in biochemical molecules, nor what happens when certain ligands (external molecules that interact with an atom or molecule) interact with those metals.

The Hard Soft Acid Base theory, or HSAB theory, states that elements differ in a property called hardness or softness, and that atoms are better able to interact with each other if they have similar degrees of hardness or softness.

Under this theory, oxygen, sodium, and potassium are all considered "hard", and indeed it is observed that the best chelating agents for sodium or potassium uses oxygen atoms to coordinate the metals.

Magnesium and calcium are considered "hard", and indeed, the magnesium in chlorophyll is chelated by nitrogen atoms, which are considered medium-hard. Iron, copper, and zinc are considered intermediate, neither hard nor soft, and they tend most often to be chelated by both medium-hard nitrogen and medium-soft sulfur.

While HSAB theory can be helpful to estimate some atomic and molecular behaviors, it does not correctly predict every interatomic force and is at best an oversimplification.

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