Usually in chemistry, the transition metals are often forgotten. In fact, there is a numbering system for the periodic table that skips them entirely! We tend to focus on the "sides" of the table, but this time, we are going to be focused on these forgotten metals.
Metals (mainly transition metals) are able to form molecules called coordination complexes, which then form coordination compounds. In this article, we will be looking at these two groups and learning what they are, their types, and how they are classified.
- This article covers coordination compounds.
- First, we will define coordination compound and coordination complex.
- Next, we will look at some examples of coordination compounds.
- Then, we will learn about the different ways coordination compounds are classified and look at examples of each.
- After that, we will learn how coordination complexes are formed through Lewis acid-base reactions.
- Lastly, we will cover some of the uses of coordination compounds.
Coordination compounds definition
A coordination compound is a compound containing a coordination complex.
A coordination complex is a molecule where the center is a metal atom bonded to several other. atoms/molecules called ligands
A complex ion is a coordination complex with a charge.
Usually, "M" is a transition metal (transition metals are located in the center of the periodic table). However, there are some cases where "M" can be other metals, such as alkali metals (group 1).
When we write the coordination compounds, the complex (metal + ligands) will always be in brackets. This is also referred to as the coordination sphere. If the complex is charged, the charge will be indicated next to (outside) these brackets.
Coordination compounds examples
Now that we know what a coordination compound is, let's look at some examples.
First, here are some examples with the same molybdenum (Mo) complex:
Fig.1-Phosphorus coordination compounds with molybdenum coordination complex(es)
Here, molybdenum is our metal and has 3 (initial) ligands. The complex then bonded with the phosphorus compound to form a coordination compound.
Sometimes, coordination compounds form for charge balancing reasons, like the one shown below:
Fig.2-Potassium (K) coordination compound
The large complex ion on the right is an anion (negatively charged), so the potassium cation (positive ion) bonds with it to balance this charge
Types of coordination compounds
There are two ways we classify coordination compounds, as shown below:
Fig.3-Chart for Classifying coordination compounds
When we categorize these compounds, we categorize based on ligand type and complex ion charge. This means we could have, for example, a homoleptic anion compound.
In a homoleptic complex, the ligands are of the same type.
In a heteroleptic complex, the ligands are of different types.
First up, here is a homoleptic coordination compound:
Fig.4-All the ligands are of the same type: cyanide (CN)
In this complex, we have a cobalt center (Co) with multiple cyanide (CN) ligands. This complex is also an anion, as shown by the 3- charge near cobalt. The potassium is there for charge balanceNext, we have a compound with heteroleptic complex:
Fig.5-Rhodium complex with different ligands
Here, our rhodium center (Rh) is bonded to two different ligands: ammonium (NH3) and chlorine (Cl). This complex is also a cation, as shown by the 2+ near the bracket. The chlorine ions are there for charge balance. The last type of coordination compound is one where the complex is neutral, as shown below:
Fig.6-Nickel (Ni) neutral complex
Nickel (Ni) is our center metal atom, bonded to four like ligands (CO), so this compound is also homoleptic
Properties of Coordination Compounds
Due to their being many different kinds of coordination compounds, they only have a few common properties, these are:
- Coloration: Coordination compounds containing transition metals are often colored since the unpaired electrons can absorb light during transitions
- Magnetism: Because they have a metallic center and unpaired electrons, coordination compounds are magnetic
- Reactivity: Coordination compounds are highly reactive
Formation of coordination complexes
Coordination complexes are the result of Lewis acid-base reactions.
A Lewis acid is a species that accepts a pair of electrons. In a coordination complex, this is the metal atom/ion.
A Lewis base is a species that donates a pair of electrons. In a coordination complex, this is the ligand(s).
Essentially, the ligand donates a pair of electrons to the metal atom, forming a sigma (σ) bond. The number of metal-ligand σ-bonds is called the coordination number.
This process repeats with each ligand to form the coordination complex.
Below is an example of this process:
Fig.7-The formation of a zinc-cyanide complex ion
The cyanide ion (CN-) donates a pair of electrons to the zinc cation (Zn2+). This process repeats four times until we have the complex ion on the left. Since there are 4 bonds here, the coordination number is 4.
The complex ion has a charge of -2, since 4 cyanide ions with a charge of -1 were added to the zinc ion with a charge of +2 (2-4=-2).
Uses of coordination compounds
Coordination compounds have a plethora of uses. Some examples are:
- Can act as enzymes or catalysts.
- Can be used to extract metals from their ores, such as nickel (Ni) and copper (Cu).
- Can be used to identify the presence of metals in a sample.
- Important for bodily functions.
- Vitamin B12.
- Part of hemoglobin.
- Aid in a plant's life.
As you can see, these compounds have a wide variety of uses, which is why there are so important!
Coordination Compounds - Key takeaways
- A coordination compound is a compound containing a coordination complex
A coordination complex is a molecule where the center is a metal atom bonded to several other atoms/molecules called ligands
A complex ion is a coordination complex with a charge
The general formula for a coordination compound is: A[M(L)x]
We can classify coordination compounds by ligand:
We can also classify coordination compounds by ion type
Coordination complexes are the result of Lewis acid-base reactions
A Lewis acid is a species that accepts a pair of electrons. In a coordination complex, this is the metal atom/ion
A Lewis base is a species that donates a pair of electrons. In a coordination complex, this is the ligand(s)
References
- Fig.1-Phosphorus coordination compounds with molybdenum coordination complex(es) (https://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Phosphenium_coordination_to_Mo_complexes.png/640px-Phosphenium_coordination_to_Mo_complexes.png) by Treverb licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/)
- Fig.2-Potassium (K) coordination compound (https://upload.wikimedia.org/wikipedia/commons/thumb/2/27/KTp%2A.png/640px-KTp%2A.png) by Smokefoot (https://commons.wikimedia.org/wiki/User:Smokefoot) licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/)
- Fig.5-Rhodium complex with different ligands (https://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/%28RhA5Cl%29Cl2.png/640px-%28RhA5Cl%29Cl2.png) by Smokefoot (https://commons.wikimedia.org/wiki/User:Smokefoot) licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/)
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