S18E3 - 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.


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 (↿⇂).


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


Now, we can take a closer look at the complex ion, 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.


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.


Common Coordination Numbers

The most common coordination numbers are 2, 4, and 6, as shown in the image below:

Common Coordination Numbers



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


As you can see in (b) and (c) above, bidentate and polydentate ligands are "chelating ligands" or chelates.

Chelate = Greek word for "claw."


Ligand Examples

Below are three examples of some common ligands, showing their covalent coordinate bonds to the metal center:

Ligand Examples 

Next up in our discussion of SECTION 18 - Transition Metals and Coordination Chemistry,

We'll cover Naming Coordination Compounds and Isomerism of Complex Ions...