Complex ions are special molecules that have a metal or its ion in the middle. These metal ions are surrounded by other molecules called ligands. The shape of complex ions is determined by these ligands, which make coordinate bonds with the metal ion.

Now, you might be thinking, "What in the world is a coordinate bond?" But don't worry, it's not as complicated as it sounds!

In this article, we'll explore how coordinate bonds affect the shapes of complex ions. We'll also discuss the different types of ligands that can be found in complex ions, and the four shapes that transition metal complexes can take. Plus, we'll take a close look at octahedral and square planar complex ions, which have a special property called stereoisomerism. And, if you're feeling brave, we'll even teach you how to draw structures of complex ions! By understanding the shapes of complex ions, we can unlock a whole new world of chemistry. So let's dive in and discover the fascinating world of complex ions together!

Complex metal ions

In chemistry, a complex metal ion is a metal ion at the center, surrounded by other molecules called ligands. These ligands have one or more pairs of electrons that they can donate to the metal ion.

Ligands can be simple, like water, or complex, like ethylenediamine or the ethanedioate ion. Regardless of their complexity, ligands donate nonbonding electrons to the metal ion. These complexes are formed when the metal ion and ligands bond together through a process known as coordinate bonding. This type of bonding is essential to the formation of complex ions and plays a crucial role in many chemical reactions. By understanding the basics of complex ions and coordinate bonding, we can begin to unravel the mysteries of chemistry and better appreciate the beauty of the chemical world around us.

Bonding in transition metal complexes

Transition metal ions have a partially filled 3d sublevel, which allows them to accept lone pairs of electrons from ligands to form coordinate bonds. The metal acts as a Lewis acid by accepting a pair of electrons, and the ligand acts as a Lewis base by donating an electron pair. Coordinate bonds are also called dative covalent bonds. For example, aluminium has the electronic configuration 1s2 2s2 2p6 3s2 3p1. When it forms an Al3+ ion, its electronic structure is now 1s2 2s2 2p6, leaving all orbitals in the 3rd level empty. Al3+ can now accept lone pairs of electrons from a molecule/ion to stabilise it. We write the formula for this complex as [ Al (H2O)6 ]3+, and this has a coordination number of 6. Coordination number tells us the number of coordinate bonds in a complex ion, and it also determines the shape of complex ions.

There are different types of ligands which donate different numbers of lone pairs of electrons. Unidentate ligands donate one lone pair of electrons, such as water, chloride ions and ammonia molecules. Bidentate ligands donate two lone pairs of electrons, such as ethylene diamine. Polydentate ligands donate more than two lone pairs of electrons, such as EDTA.

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In complex ions, the neutral ligands form positive complexes, while negative ligands form negative complexes. The overall charge of a complex is determined by the oxidation state of the metal ion.

Bidentate ligands are ligands that form two coordinate bonds, meaning they donate two lone pairs of electrons. Examples of bidentate ligands include 1,2-diaminoethane (en) and the ethanedioate ion. These ligands form complexes called chelates, which are shaped like crabs. Chelating agents are ligands that can form multiple coordinate bonds with a metal ion. Multidentate ligands can donate three or more lone pairs of electrons. The EDTA4- ion is a multidentate ligand that can form six bonds with a metal ion. It is also a chelating agent and is used to cure metal poisoning and in many cleaning products to remove calcium ions from hard water.

The type and number of ligands in a complex ion determine its shape. The most common shapes for complex ions are tetrahedral, square planar, octahedral, and linear. For example, a complex with four ligands usually has a tetrahedral shape, while a complex with six ligands usually has an octahedral shape. In summary, understanding the different types of ligands and their ability to form coordinate bonds is essential in understanding the formation, properties, and applications of complex ions. The shape of complex ions is determined by the type and number of ligands present, and this has significant implications for their properties and reactivity.

Common complex shapes of metal ions

Overall, there are four complex shapes of transition metal ions:

Linear Square planar Tetrahedral Octahedral

The two most common ones are tetrahedral and octahedral.

Tetrahedral complex ions

Tetrahedral complexes have four coordinate bonds with bond angles of 109.5º. Large ligands like Cl- form tetrahedral complexes.

Two examples of tetrahedral complexes are copper chloride [CuCl4]2-, and cobalt chloride [CoCl4]2-, shown below.

Tetrahedral complex ions, chemguide

Notice how four Cl- ions form a bond with the central metal ion. Both Cu (II) and Co (II)  have a 2+ charge. Each Cl ion carries a 1- charge, thus this gives the overall charge of 2- on both complex ions.

(4 X Cl- = 4- ; 4- + 2+ = 2- overall charge)

Octahedral complex ions

Octahedral complexes form when there are six coordinate bonds formed around the central metal. They have a bond angle of 90º.

Small ligands like H2O or NH3 form octahedral complexes. A small ligand like water or ammonia leaves space for more ligands to fit around the complex ion. On the other hand, large ligands like chlorine leave space only for four ligands.

There are many ways these six coordinate bonds can be formed. Maybe you have already thought of a few! Let us take a look at the possibilities.

Six small monodentate ligands - each ligand forms one coordinate bond with the central metal. H2O and NH3 are examples of ligands that form one coordinate bond.

Octahedral complex ions, chemguide

Three bidentate ligands - each ligand forms two coordinate bonds with the central metal. The oxalate (ethanedioate) ion (ox) or ethylenediamine (en) are examples of an ion and molecule respectively, that can donate two lone pairs of electrons and form two coordinate bonds. One multidentate ligand - one ion/molecule can form three or more coordinate bonds to the metal ion. EDTA4- is the most common example of a multidentate ligand.

In addition to the two shapes we have looked at, linear and square-shaped complexes can also form. Let us consider these now.

Linear complex ions

Linear complex ions form when there are two coordinate bonds. They have a bond angle of 180°.

A common example of a linear complex ion is diamminesilver(I) [Ag(NH₃)₂]⁺, which is used in Tollens’ reagent.

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Read about Tollens’ reagent in Variable Oxidation State of Transition Elements.

Square planar complexes

Square planar complexes have four coordinate bonds too! Unlike tetrahedral complexes, they have a bond angle of 90º.

An early drug used in the treatment of cancer, cis-platin, forms a square planar complex. It has the platinum ion, Pt2+, as its central ion, and two ammonia molecules and two chloride ions as ligands.

Cis-platin, chemguide

You might have guessed from the name that cis-platin is a geometric isomer. Square planar and tetrahedral complexes are special because they can show stereoisomerism. We will explore what that means next.

Isomerism in transition metal complexes

Occasionally, we see stereoisomerism in octahedral and square planar complexes. Stereoisomers are molecules with the same order of atoms, but different spatial arrangements. There are two types of stereoisomerism: geometric E-Z isomerism and optical isomerism. Let us examine how they display in metal complexes.

Find out more about this in Isomerism.

Isomerism in transition metal complexes: Geometric isomerism

Square planar complexes can exhibit geometric cis-trans isomerism, where high-priority ligands are either adjacent to each other or across from each other. For example, cis-platin and trans-platin are geometric isomers, with the chlorine and ammonia ligands next to each other in the cis-isomer and opposite each other in the trans-isomer.

Octahedral complexes with monodentate ligands can also exhibit geometric isomerism, with two ligands being different from the other four. For example, cis and trans-tetraamminedichlorocobalt(III) ions are octahedral geometric isomers, with the chlorine ions adjacent to each other in the cis-isomer and opposite each other in the trans-isomer. Geometric isomerism has important implications for the properties and reactivity of complex ions. For example, cisplatin is used as a chemotherapy drug to treat cancer, while transplatin is less active. Understanding and controlling geometric isomerism is therefore crucial in the design and development of new drugs and materials.

Isomerism in transition metal complexes: Optical isomerism

Optical isomers are non-superimposable mirror images of each other, and they show no plane of symmetry. Octahedral complexes with bidentate ligands, such as ethylenediamine, can exhibit optical isomerism.

To draw the structure of a complex ion, we use wedge drawings to show 3D molecules. A solid straight line represents a bond on the same plane as the page, a solid wedge shows bonds sticking out in front of the page, and a dashed wedge represents bonds behind the plane of the page.

To deduce the overall polarity of complexes that exhibit isomerism, we need to consider the charges of the ligands within the complex and the charge on the central transition metal ion of the complex. Not all complexes have an overall charge, but when they do, it can be calculated using these factors.

Understanding the polarity of complex ions is important in understanding their properties and reactivity. For example, polar complexes are often more soluble in polar solvents and have higher boiling points than nonpolar complexes. This has important implications for their use in various applications, such as catalysis and materials science. 

Tetrahedral complex ions, chemguide Looking at a complex that we have approached earlier. We can identify that both of these complexes have a 2- minus charge. We will be exploring how we got to this final overall charge. For [CuCl4]2- we can first identify that the ligands are Cl-. As there are 4 x Cl-, this means that together the ligands have a 4- charge. For the central transition metal ion, we can now determine that it has a 2+ charge. This is because in order to go from the 4- charge from the ligands to the 2- overall charge, we need the transition ion to be 2+. So overall: the 4 x Cl- ligands give a 4- charge which added to the Cu2+ charge gives the overall complex the charge of 2-.We can then explore [CoCl4]2-. This is similar to the previous example, whereby the 4 x Cl- ligands give a 4- charge and Co has a 2+ charge, giving the overall charge of the complex ion 2- .

Shapes of Complex Ions - Key takeaways Complex ions have a metal ion in their centre, with several ligands or molecules surrounding it. The molecules or ions surrounding the central metal are called ligands. There are four shapes a complex ion can form, but the two most common are octahedral and tetrahedral. A tetrahedral complex shape is formed when there are four coordinate bonds. An octahedral complex shape is formed when there are six coordinate bonds formed around the central metal. Complex ions can show two forms of isomerism: geometrical and optical. Geometrical isomerism is when a complex ion can rearrange itself in two different ways (cis and trans) around the central transition metal. For a complex ion to show optical isomerism, they must have non-superimposable mirror images.

Shapes of Complex Ions

What is the geometry of a complex ion?

The geometry of a complex ion is how the complex ion is shaped. There are four possible complex geometries or shapes of metal ions: Tetrahedral Octahedral Square planar Linear

How do you tell if a complex ion is tetrahedral or square planar?

Tetrahedral complexes have four coordinate bonds and bond angles of 109.5º. Square planar complex ions also have four coordinate bonds, but bond angles of 90°.

How do you determine the shape of a complex ion?

The coordination number and bond angle determine the shape of complex metal ions. For example, octahedral shaped complexes have six small ligands around a metal ion and a bond angle of 90º.

How do you draw complex ions?

To draw a complex ion, you must first identify its shape. For example, octahedral complexes have six ligands. Four of them lie in a plane and the other two are perpendicular above and below the plane. We draw the four planar ligands with two wedged dashed lines and two solid wedged lines. You draw the other two ligands above and below the central metal ion. Learning the shapes of complex ions will allow you to draw each different complex ion, even if it is made from different transition metals and ligands.

Can magnetic moment tell the shape of a complex ion?

Yes. The magnetic moment can help you infer the geometry of a complex ion. Essentially, chemists use magnetic moment along with spectroscopy to tell the oxidation number and stereochemistry of the central metal in a coordination complex. For example, they might observe the weight difference of a metal complex in the presence and absence of a magnetic field.

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