Shapes of Complex Ions

Complex ions are molecules with a transition metal or its ion at the centre, surrounded by other molecules called ligands. Ligands determine the shapes of complex ions by forming coordinate bonds. 

Shapes of Complex Ions Shapes of Complex Ions

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Table of contents

    Sound complicated? Not to worry, complex ions will not seem complicated anymore once you have read this!

    • We will learn how coordinate bonds determine the shapes of complex ions.
    • We will explore the different types of ligands in complex ions, and consider the four shapes transition metal complexes can take.
    • We will also discover how octahedral and square planar complex ions show stereoisomerism.
    • Finally, we will learn how to draw structures of complex ions.

    Complex metal ions

    A complex metal ion is a central metal ion surrounded by ligands. Ligands are species that can donate one or more lone pairs of electrons.

    This means ligands donate a pair of nonbonding electrons. Ligands can be very simple like the water molecule, or they can be much more complex, like ethylenediamine (en) or the ethanedioate ion (ox).

    Shapes of Complex Ions, simple and complex ligands, StudySmarterFig. 1 - Simple and complex ligands

    Complexes consist of a transition metal or its ion bonded coordinately to ligands. Let us take a closer look at what we mean by coordinate bonding.

    Bonding in transition metal complexes

    Transition metal ions have a partially filled 3d sublevel. Ligands form coordinate bonds when an orbital that contains a lone pair of electrons overlaps with a vacant orbital on the metal ion. In other words, 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.

    Shapes of Complex Ions, aluminium electronic configuration 1s2 2s2 2p6 3s2 3p1, StudySmarterFig. 2 - Aluminium electron configuration

    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. In the example below, Al3+ accepts six lone pairs from six water molecules.

    Shapes of Complex Ions, hexaquaaluminium aluminium surrounded by six water ligands that each donate a lone pair, StudySmarterFig. 3 - Hexaquaaluminium

    We write the formula for this complex as [ Al (H2O)6 ]3+. Six water ligands each give the aluminium ion one lone pair of electrons. So, we say the complex [Al (H2O)6 ]3+ has a coordination number of 6.

    Coordination number tells us the number of coordinate bonds in a complex ion. Coordination numbers also determine the shape of complex ions. Keep reading to learn more!

    Before we consider the shape of complex ions, let us briefly discuss the different types of ligands. We classify ligands by how many coordinate bonds they can form.

    Unidentate ligands donate one lone pair of electrons.

    Water, chloride ions and ammonia molecules are examples of unidentate ligands.

    Shapes of Complex Ions, Unidentate ligands water,, chlorine ion and ammonia, StudySmarterFig. 4 - Complex ions with unidentate ligands

    Notice how the neutral ligands form positive complex ions whilst negative ligands form negative complexes. Another thing you might notice is how the oxidation state of the ion determines the overall charge of the complex.

    Bidentate ligands form two coordinate bonds, meaning they donate two lone pairs of electrons.

    The two molecules below, 1,2-diaminoethane (or ‘en’ for short) and the ethanedioate ion are bidentate ligands.

    Shapes of Complex Ions, bidentate ligands ethanedioate and 1,2-diaminoethane, StudySmarterFig. 5 - Bidentate ligands

    These ligands form complexes called chelates, meaning ‘crab-like’. Notice how the ligands are shaped like crabs? We call these types of ligands chelating agents.

    Multidentate ligands (also called polydentate) 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.

    Shapes of Complex Ions, Multidentate EDTA ion, StudySmarterFig. 6 - Multidentate ligand

    You can see from its shape that EDTA4- is also a chelating agent. It forms a complex by surrounding the metal ion. This gives it many useful applications! For example, it can be used to cure metal poisoning. Many cleaning products contain EDTA to help remove calcium ions from hard water.

    Now, let us see how these ligands influence the shape of metal complexes.

    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.

    Shapes of Complex Ions, tetrahedral complexes copper chloride and cobalt chloride, StudySmarterFig. 7 - Tetrahedral complex ions

    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.

    Shapes of complex ions, Octahedral ions with monodentate water and ammonia ligands, StudySmarterFig. 8 - Octahedral complex ions

    • 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.

    Shapes of Complex Ions, linear diamminesilver(I), StudySmarterFig. 9 - Linear complex ion

    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.

    Shapes of Complex Ions, cis-platin, StudySmarterFig. 10 - Cis-platin

    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 display geometric cis-trans isomerism (also called E/Z isomerism). These complexes have their high-priority ligands either adjacent to each other, or across from each other. For example, cis-platin and trans-platin are geometric isomers.

    Shapes of Complex Ions, geometric isomers square planar complexes cis/trans-platin, StudySmarterFig. 11 - Cis and trans platin

    Notice how cis-platin has the chlorine and ammonia ligands right next to each other. On the other hand, trans-platin has the chlorine and ammonia ligands opposite each other.

    The isomer with the high priority ligands adjacent to each other is the cis-isomer, in this case, cis-platin. The isomer with the high priority ligands across from each other is the trans-isomer.

    We can also see geometric isomerism in octahedral complex ions with monodentate ligands. Two of the ligands must be different from the other four. For example, cis and trans-tetraamminedichlorocobalt(III) ions shown below are octahedral geometric isomers.

    Shapes of Complex Ions, octahedral geometric isomers, StudySmarterFig. 12 - Cis and trans-tetraamminedichlorocobalt(III) ion

    You will notice how the two chlorine ions are adjacent to each other in the cis-isomer, while they are opposite each other in the trans-isomer.

    Isomerism in transition metal complexes: Optical isomerism

    Optical isomers are non-superimposable mirror images of each other. They show no plane of symmetry. We can see this type of stereoisomerism in octahedral complexes with bidentate ligands. Remember, bidentate means they donate two lone pairs of electrons.

    Ethylenediamine (or ‘en’ for short) is an octahedral complex ion that shows optical isomerism. You might not be able to tell by looking at the structural formulas below, so making some physical models might be helpful!

    Ethylenediamine is also called 1,2-diaminoethane.

    Shapes of Complex Ions octahedral optical isomer ethylenediamine StudySmarterFig. 13 - Cis and trans-tetraamminedichlorocobalt(III) ion

    With a little imagination, you can see that both molecules are mirror images of each other. But you cannot superimpose them no matter which way you turn them.

    Before we conclude, let us take a look at how you might draw the structure of a complex ion.

    How to draw complex ions

    We use wedge drawings to show 3D molecules. Here is how they work:

    • A solid straight line shows a bond on the same plane as the page.
    • A solid wedge represents bonds sticking out in front of the page.
    • A dashed wedge shows bonds behind the plane of the page.

    Take a look at the examples below. When you know the rules of wedge drawings, you can draw any complex molecule, as long as you know its shape!

    Shapes of Complex Ions, diagram shows how to draw complex shapes of ions, StudySmarterFig. 14 - How to draw complex shapes of ions

    How can we deduce the overall polarity of complexes that show isomerism?

    You may notice that ligands are within two square brackets, and you also may notice that sometimes outside the brackets there is charge. It is important to remember that only some complexes may have an overall charge and this can be calculated using two things:

    1. The charge of the ligands within the complex.
    2. The charge on the central transition metal ion of the complex

    Looking at a complex that we have approached earlier.

    Shapes of Complex Ions, tetrahedral complexes copper chloride and cobalt chloride, StudySmarterFig. 15 - Tetrahedral complex ions

    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.
    Frequently Asked Questions about 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:

    1. Tetrahedral
    2. Octahedral
    3. Square planar
    4. 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.

    Test your knowledge with multiple choice flashcards

    How many coordinate bonds do tetrahedral complexes have?

    How many coordinate bonds do octahedral complexes have?

    What is the bond angle of tetrahedral complexes?

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