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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.
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Team Shapes of Complex Ions Teachers
Sound complicated? Not to worry, complex ions will not seem complicated anymore once you have read this!
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).
Fig. 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.
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.
Fig. 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.
Fig. 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.
Fig. 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.
Fig. 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.
Fig. 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.
Overall, there are four complex shapes of transition metal ions:
The two most common ones are tetrahedral and octahedral.
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.
Fig. 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 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.
Fig. 8 - Octahedral complex ions
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 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.
Fig. 9 - Linear complex ion
Read about Tollens’ reagent in Variable Oxidation State of Transition Elements.
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.
Fig. 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.
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.
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.
Fig. 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.
Fig. 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.
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.
Fig. 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.
We use wedge drawings to show 3D molecules. Here is how they work:
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!
Fig. 14 - How to draw complex shapes of ions
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:
Looking at a complex that we have approached earlier.
Fig. 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- .
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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:
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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