Protoporfyrine IX
A ligand is a molecule or ion that has a free electron pair that can be used to bind with a metal or a metal ion. The metal ligand bond can be described as the interaction between a lewis base and a lewis acid. Ligand is derived from the Latin "ligar", which means binding.
In ligands, a distinction is made between:
The figure below shows the structural nucleus of the tetradentate ligand found in compounds such as hemoglobin and leaf green. The free electron pair on the central nitrogen atoms (possibly after the release of the hydrogen atoms) are the electrons that provide complex formation. The example of this group of compounds is Ethylenediaminetetraacetic acid, which is commonly known as the abbreviation EDTA.
Ligands occur as part of complex ions or molecules in solutions, but they can also be incorporated into solids. Water also occurs often as a ligand. In a crystal structure, one speaks of crystal water. Also see
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