What Is a Complex Ion? — Definition & Overview

Let's discuss complex ions!!

A complex ion is a Lewis acid-base complex that contains a central metal ion with a number of other ions or molecules that surround it.

The bonds that forms between the metal ion and the surrounding ligands are known as Dative bonds. These are covalent bonds in which both electrons come from the same atom. In this case, both electrons in the pair are attracted by both of the nuclei.

The ions or molecules that are attached to the metal are called ligands, and the number of these ligands that are bound is known as the coordination number. More specifically, the coordination number is the number of donor atoms that can be bound to the metal center.

The coordination number of a complex is influenced by the relative sizes of both the metal ions and of the ligands, and also by electronic factors. Typically, the larger the charge on the central ion is, the more attraction there will be for negatively charged ligands. At the same time, as the charge becomes bigger, there number of groups able to coordinate becomes limited.

Many ligands are known as monodentate, because they have just a single donor atom that can attach to the metal.

Other ligands are able to attach to the metal in more than one place, known as polydentate or chelating ligands.

A bidentate ligand is one that contains a pair of nonbonding electrons that can form a covalent bond to the metal.

A tridentate ligand has three atoms that can function as donor atoms in a coordination complex.

Tetradentate ligands have four atoms hexadentate ligands have six.

The chelate effect is the enhanced affinity of chelating ligands for a metal ion in comparison to the affinity of a collection of similar monodentate ligands for the same metal.

Examples of common ligands are shown below, with the M representing the metal center to which they would attach:

Most ligands are either $neutral$, such as carbon monoxide ($CO$), or they can be $anionic$, like $Cl^{-}$. The neutral ligands are independently stable in their free states, meaning they don't need to be out searching for another ion to be satisfied. However, anionic ligands, those that are negatively charged in their free state, are stabilized only when they are attached to a central metal ion.

The table below contains a number of representative ligands, some of which are neutral and some anionic:

$$\begin{array}{|c|c|c|} \hline \text{*Name*}&Abbreviation&Formula\\ \hline \text{ammine}& &NH_{3}\\ \hline \text{aqua}&aq&H_{2}O\\ \hline \text{bipyridine}&bipy&C_{10}H_{8}N_{2}\\ \hline \text{carbonyl}& &CO\\ \hline \text{chloro}& &Cl^{-}\\ \hline \text{cyano}& &CN^{-}\\ \hline \text{cylopentadienyl}&Cp&C_{5}H_{5}^{-}\\ \hline \text{hydrido}& &H^{-}\\ \hline \text{methyl}&Me&CH_{3}^{-}\\ \hline \text{thiocyanato}& &SCN^{-}\\ \hline \end{array}$$

What all of these ligands have in common is that they have active lone pairs of electrons in the outer energy level that are used to form co-ordinate bonds with the metal ion. A co-ordinate bond is a covalent bond where both electrons come from the same atom. Since all ligands are lone pair donors, we can say that they function as Lewis bases.

As for the metal center, some main group elements will form these complexes, such as aluminum, lead, and tin, but those with transition elements are particularly important in inorganic chemistry. Complex ions that consist of d-block transition metals and ligands are often referred to as coordination compounds. These transition-metal complexes most commonly have a coordination number of 2, 4, or 6, but they have have been seen ranging from 1 to 12. Thee metal ions are positively charged and function as Lewis acids. Small, highly charged metal ions, such as $CU^{2+}$ or $Ru{3+}, have the greatest tendency to act as Lewis acids and therefore have the greatest likelihood of forming complex ions.

Examples of common complexes with their coordination numbers can be seen below:

The charge on the complex will always just be the sum of the charges on the individual ions or molecules that form it. Also, note that the coordination number of a complex will tend to increase as the charge on the metal ion becomes larger.

Well, now you know what a complex ion is! To learn how they are formed, check out this link here

(Video) Complex Ions, Ligands, & Coordination Compounds, Basic Introduction Chemistry

by The Organic Chemistry Tutor

In this video, The Organic Chemistry Tutor explains complex ions. He starts by explaining what a complex ion is. Then you'll learn some of the basic vocabularies. Finally, he explains how complex ions can form compounds. After watching this video, you'll understand complex ions!