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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…
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Jetzt kostenlos anmeldenUsually 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.
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).
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:
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:
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
There are two ways we classify coordination compounds, as shown below:
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.
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:
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:
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:
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).
Coordination compounds have a plethora of uses. Some examples are:
As you can see, these compounds have a wide variety of uses, which is why there are so important!
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:
homoleptic complex: the ligands are of the same type
heteroleptic complex: the ligands are of different types
We can also classify coordination compounds by ion type
Anion
Cation
Neutral
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)
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