Coordination compounds hgands preferred 68

Elements with very low electronegativity (alkali metals, alkaline earth metals, such as Na, Ca and Mg) and elements with high electronegativity (halogens such as Cl and I) occur mainly as free ions in biological materials, and are preferably involved in electrostatic interactions. However, even these elements can form less soluble compounds (calcium oxalate), covalent compounds (hormones thyroxine and triiodothyronine are iodinated aromatic amino acids, see Section 2.2.1.2.5) or complex compounds (chlorides as Hgands and some metal ions as central atoms). A ligand is an entity (atom, ion or molecule), which can act as an electron pair acceptor to create a coordinate covalent bond with the central ion. Cd and Hg also tend to form covalent compounds. 

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