complex ion

chemistry

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June 24, 2026

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A complex ion, also known as a coordination complex, is a compound or ion featuring a central metal atom or ion bonded to surrounding ions, groups, or molecules through coordinate bonds. These complexes can be electrically neutral or carry a positive or negative charge.12

For example, the hexaaquanickel(2+) ion, [Ni(H₂O)₆]²⁺, is an ionic coordination complex where a central nickel ion is bonded to six water molecules. Ligands, which are the surrounding ions or molecules, can be neutral (like water or ammonia), negatively charged anions (like fluoride or cyanide), or occasionally positively charged cations.2

Complex ions can exist freely in solutions or be part of crystalline salts. They are fundamental in understanding chemical bonding and exhibit diverse properties like color and magnetic susceptibility.23

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zinc group element
Chemical reactivity
…ionic, but cadmium also forms complex ions with ligands (atoms, ions, or molecules that donate electrons to a central metal ion); e.g., the complex ion with ammonia NH₃, having the formula [Cd(NH₃)₄]²⁺, or with the cyanide ion, the formula [Cd(CN)₄]²⁻. Differing from zinc and mercury, cadmium can form the complex…
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…ionic, but cadmium also forms complex ions with ligands (atoms, ions, or molecules that donate electrons to a central metal ion); e.g., the complex ion with ammonia NH₃, having the formula [Cd(NH₃)₄]²⁺, or with the cyanide ion, the formula [Cd(CN)₄]²⁻. Differing from zinc and mercury, cadmium can form the complex…
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coordination compound
Characteristics of coordination compounds
Complex ions—that is, the ionic members of the family of coordination substances—may exist as free ions in solution, or they may be incorporated into crystalline materials (salts) with other ions of opposite charge. In such salts, the complex ion may be either the cationic (positively…
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Complex ions—that is, the ionic members of the family of coordination substances—may exist as free ions in solution, or they may be incorporated into crystalline materials (salts) with other ions of opposite charge. In such salts, the complex ion may be either the cationic (positively…
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ion-exchange reaction
In the laboratory
…whose anions form negatively charged complexes with the lanthanoid ions are used to wash the ions from the column. The metal ions themselves are held by the resin; the complexes are not. Those ions that form more stable complexes do not adhere to the resin and therefore move off the…
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…whose anions form negatively charged complexes with the lanthanoid ions are used to wash the ions from the column. The metal ions themselves are held by the resin; the complexes are not. Those ions that form more stable complexes do not adhere to the resin and therefore move off the…
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