Complex Ion

A complex ion is an ion that contains one or more ligands that are attached to a central metal cation through a dative covalent bond.

A ligand is a species that can form a dative covalent bond with a transition metal using its lone pair of electrons. H₂O, NH₃, Cl^–, OH^–, and CN^– are examples of ligands.

Table of Contents

• Bonding in Complex Ions
• Shape of Complex Ion
• Type of Complexes
• Nomenclature of Complex Ions
• Applications of Complex Ion
• Frequently Asked Questions – FAQs

Bonding in Complex Ions

The coordination number is the number of ligand-binding sites on the metal ion. The bond between the metal ion and the ligand, where the ligand supplies both electrons, is known as a co-ordinate covalent bond.

A co-ordinate bond (also known as a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons originate from the same atom.

Shape of Complex Ion

Complex ion shapes includes octahedral, tetrahedral, and linear complexes. The shape of the complex varies depending on the type of ligand and the coordination number.

• Octahedral – The most common shape is octahedral. It happens when there are six ligands.
• Tetrahedral – Tetrahedral is the next most common. It occurs in complexes with Cl- ligands because only four of them can fit around the metal ion, so this is the configuration they adopt.
• Linear – Ions of silver (I) will form linear complexes. This is when one ligand is on either side of the silver ion, with an angle of 180° between them.

Types of Complexes

Homoleptic complexes

Homoleptic complexes are those in which the metal atom or ion is linked to only one type of donor atom, for example, [Co(NH₃)₆]³⁺.

Heteroleptic complexes

Heteroleptic complexes are those in which the metal atom or ion is linked to more than one type of donor atom, for example, [Co(NH₃)₄CI₂]⁺.

Labile and Inert complexes

Complexes with fast ligand substitution are known as labile complexes, while complexes with slow ligand substitution are known as inert complexes.

Nomenclature of Complex Ions

Complex ions have a slightly different nomenclature: The metal is referred to as the central metal ion. The anions or molecules that are attached to the metal are referred to as ligands. The coordination number is the number of ligand-binding sites on the metal ion.

When naming a complex ion, the ligands come first, followed by the metal ion.

Naming the ligands-

If there is more than one ligand, the standard prefixes apply.

• There is no issue for a complex ion containing only one type of ligand.
  For example –

• When an ion contains more than one type of ligand, the ligands are named alphabetically, ignoring prefixes.
  For example –

Naming the Metal

• A cationic complex is a positively charged complex ion. The metal, in this case, is named by the addition of its oxidation state. For example– [Cu(H₂O)₆]²⁺ is named as hexaaquacopper(II) ion because copper’s oxidation state is +2.
• An anionic complex is a complex ion that is negatively charged. In this case, the metal’s name has been changed to reflect the fact that it has become a negative ion. The ending -ate demonstrates this.
  For example – cobalt becomes cobaltate, aluminium becomes aluminate, chromium becomes chromate, copper becomes cuprate, iron becomes ferrate[Al(H₂O)₂(OH)₄]^– is called the diaqua tetrahydroxoaluminate ion.

Applications of Complex Ion

• Iron exists in the blood. The protein haemoglobin contains an iron atom that is covalently bonded to four nitrogen atoms from larger molecules. Oxygen can be transported because it coordinates with the Fe²⁺ ion. Carbon monoxide, unlike oxygen, forms a more stable complex. As a result, oxygen uptake is inhibited, and carbon monoxide poisoning may occur.
• A platinum complex ion is used in the anti-cancer drug Cisplatin: it consists of two ammonia and two chloride ligands on platinum.
• Different silver complexes have practical applications as well.
  [Ag(NH₃)₂]⁺ is used in Tollens’ reagent,
  [Ag(CN)₂]^– is used as the electrolyte in silver plating.