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Complex Ions and Coordination Compounds

SECTION 18 - Transition Metals and Coordination Chemistry


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.

Coordination Compound Example

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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...)

Complex Ion Example

[ 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 +3Why?  Because of 3 negatively charged Cl- ions and 5 neutral NH3 ligands.

The coordination number is 6Why?  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:

Common Coordination Numbers

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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")...

Bidentate Ligands Examples

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.

EDTA Polydentate Ligand

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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:

Ligand Examples 
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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...