What is a complex ion?

A complex ion is a chemical species formed by a central metallic atom, often with a positive oxidation state, which is linked by a group of neutral molecules or negative ions, through dative or coordinated covalent bonds. The central metal atom is called the coordination center (which is why these are said to be a type of coordination complex), while the molecules or ions that donate the pairs of electrons from the coordinated bonds are called ligands.

Depending on the electrical charge of the metal center and that of the ligands, complex ions can be positive (complex cations) or negative (complex anions).

Complex ions are much more common than we can imagine and are part of complex salts. In fact, its presence in a salt is precisely what classifies it as a complex salt.

Characteristics of complex ions

Some characteristic features of these chemical species are:

• The central metal atom is almost always a transition metal (d block) or an internal transition metal (f block, made up of the lanthanides and actinides). In some cases they may be p-block metals.
• They can exist in solution or as complex salts in the solid state.
• Because the central atom receives a pair of electrons in a dative covalent bond, it behaves like a Lewis acid.
• The ligands can be neutral molecules such as water (H ₂ O) or ammonia (NH ₃ ) or they can be mono- or polyatomic anions such as chloride ion (Cl ^– ) or cyanide ion (CN ^– ).
• Ligands must always have atoms with unshared lone pairs of electrons as is the case with many compounds containing oxygen, nitrogen, sulfur, and phosphorus.
• The ligands, being electron pair donors, are Lewis bases.
• The same ligand can contribute more than one pair of electrons through more than one atom with lone pairs of electrons, in which case it is usually called a chelating agent, as in the case of the ethylenediaminetetraacetate or EDTA 4- ^ion .

Types of complex ions

Depending on the particular characteristics of the metal center and the ligands that surround it, some different types of complex ions can be distinguished:

complex cations

They are complex ions with a positive charge.

complex anions

They are complex ions with negative charges.

Mononuclear complex ions with monodentate ligands

They are the most common and refers to those that only contain a metallic center which is surrounded by ligands that only contribute a pair of electrons each. This type of ligand is called a monodentate ligand.

Chelates: mononuclear complex ions with polydentate ligands

They are those in which a single metal center is linked to a single ligand, but through multiple coordinate bonds. Ligands with several donor atoms are called polydentate ligands or chelating agents, and the complexes they form with different metals are called chelates. If the complex has an electric charge it will also be a complex ion.

polynuclear complex ions

There are examples of complex ions in which several metal centers are linked together, either through a metal bond (as in the case of the [Re 2 Cl 4 ] 2- ion) or through _a bidentate _ligand that ^acts as a bridge between the two metal centers (as in the case of the ion [(NH ₃ ) ₅ ] Co – NH ₂ – Co(NH ₃ ) ₅ ] ⁵⁺ ). This type of complex is called polynuclear complexes, since they have more than one coordination center.

Examples of complex ions

As mentioned at the beginning, the complex ions can be both positive and negative, the metal center can be positive or neutral, and the ligands attached to it can be both neutral molecules and negative ions. In addition, they may contain one or more focal points. Here are some examples highlighting these features.

Example 1: The cation [(NH3)5Ru(C ₄ H ₄ N ₂ )Ru(NH ₃ ) ₅ ] ⁵⁺

This is an example of a complex polynuclear cation with neutral ligands and positive metal centers.

Example 2: The anion hexacyanoferrate (II) or [Fe(CN) ₆ ] ⁴⁺

This is an example of a mononuclear complex ion with a positive central atom, but a net negative charge.

Example 3: [Co(SO ₄ )(NH ₃ ) ₅ ] ⁺

[Co(SO ₄ )(NH ₃ ) ₅ ] ⁺ is a complex mononuclear cation that has a cobalt (III) center surrounded by 5 neutral amino ligands and a negative sulfate ligand.

Example 4: [Au ₆ C(PPh ₃ ) ₆ ] ²⁺

This is another example of a positively charged polynuclear complex ion in which a carbide anion bridges all 6 gold(I) cation metal centers.

References

Coordination compound – Ligands and chelates . (2021). Retrieved May 29, 2021, from https://www.britannica.com/science/coordination-compound/Ligands-and-chelates

Cotton, Frank Albert; Wilkinson, Geoffrey, Murillo, Carlos A.; Bochman, Manfred. (1999). Advanced inorganic chemistry. 6th ed. Wiley

Lawrance, Geoffrey A. (2010). Introduction to the Chemistry of Coordination . wiley. https://onlinelibrary.wiley.com/doi/book/10.1002/9780470687123