Coordination Compounds typically consist of a "complex ion" and its counterions.
complex ion = transition metal ion with its attached ligands.
counterions = anions or cations as needed to produce a compound with no overall net charge.
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Coordination Compounds
Transition metal cations form coordination compounds which are usually colored and often paramagnetic.
paramagnetic = have unpaired electrons (↿) and are therefore magnetic.
diamagnetic = all paired electrons (↿⇂).
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Coordination Compound Example
ex: [ Co(NH3)5Cl ] Cl2
Inside the brackets is the complex ion, Co(NH3)5Cl2+
We know it's +2 thanks to the counterion's identity.
Outside the brackets are 2 Cl- counterions, anions in this case.
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Now, we can take a closer look at the complex ion, Co(NH3)5Cl2+.
Co(NH3)5Cl2+
- has 5 NH3 ligands
- has 1 Cl- ligand
- cobalt is Co3+ (we'll learn why in a bit...)
[ Co(NH3)5Cl ] Cl2 dissolves just like any other ionic solid.
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Coordination Number
➞ represents the number of bonds formed between the metal ion and the ligands in the complex ion.
➞ not the same thing as "oxidation number" !!
ex: [ Co(NH3)5Cl ] Cl2
The oxidation number is +3. Why? Because of 3 negatively charged Cl- ions and 5 neutral NH3 ligands.
The coordination number is 6. Why? Because the Co3+ bonds to 6 total ligands: 5 -NH3 and 1 -Cl-.
There's no way to predict what the coordination number will be until you "see" the compound's formula.
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Common Coordination Numbers
The most common coordination numbers are 2, 4, and 6, as shown in the image below:
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Ligands
Ligands = neutral molecules or charged ions having a lone electron pair that bonds (coordinates) to a metal ion.
These "bonds" are often called: coordinate covalent bonds.
Types of Ligands
a) Monodentate (or unidentate) ligands.
➞ can form 1 bond to a metal atom.
Monodentate ligands have one electron pair that "adds in" to the metal ion center.
ex: H2O, NH3, CO, NO, NO2-, CN-, Br-, F-, I- Cl-, OH-
b) Bidentate ligands.
➞ can form 2 bonds to a metal atom.
Bidentate ligands have two electron pairs that "add in" to the metal ion. Two common ones are the oxalate ion ("ox"), C2O42-, and ethylenediamine ("en")...
c) Polydentate ligands.
➞ can form more than 2 bonds to a metal atom.
The most common: ethylenediaminetetraacetate, or EDTA.
EDTA = a hexadentate ligand that has 6 attachment points and thus, virtually surrounds the metal ion.
This "surrounding" of the metal ion is what makes EDTA a good scavenger in removing toxic heavy metals from the human body.
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As you can see in (b) and (c) above, bidentate and polydentate ligands are "chelating ligands" or chelates.
Chelate = Greek word for "claw."
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Ligand Examples
Below are three examples of some common ligands, showing their covalent coordinate bonds to the metal center:
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Next up in our discussion of SECTION 18 - Transition Metals and Coordination Chemistry,
We'll cover Naming Coordination Compounds and Isomerism of Complex Ions...