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CHEM1021
,CHEM1021
Bold (blue) – Week number and core focus
Italic (blue) – Pre-lecture or lecture content
Italic and underlined (black) – Heading
Bold (black) – Subheading
Week 1: Inorganic Chemistry I – Transition Metal Complexes
Pre-lecture content
Transition metals and their d-electron counts
The definition of a transition metal is ‘an element that has a partially filled d sub-shell’.
- The transition metals (with variable oxidation state usually +2 or +3) therefore include:
- Remember that a filled or a half-filled set of 3d orbitals is energetically favourable, leading to
anomalous behaviour for Cr and
Cu.
Counting d-electrons in transition metals ions
- Transition metals is that they can
exist in a variety of oxidation
states (Fe2+, Fe3+).
- Different oxidation states have
different d-electron counts.
§ This is important because
the d-electron count
, affects many chemical properties.
- A key point to remember is that in transition metal ions, the 3d orbitals are lower in energy than the 4s orbital
(the reverse of the situation with neutral transition metal atoms).
§ Hence, in a transition metal ion, the 4s orbital will usually be empty.
- The d-orbital electrons in a transition metal ion is group number (1-18) minus oxidation state (charge).
Coordination compounds
In reality, transition metal ions are rarely isolated as single atoms floating in space.
- Usually, transition metal ions are surrounded by, and bound to, a group of small molecules giving a
"coordination compound".
Let’s examine the following example:
- Consider an Fe3+ ion in aqueous solution.
- The positive charge of the Fe3+ ion will interact with the lone pairs of electrons on water molecules.
§ Bonds will form!
- In this example, the water molecules are said to be "ligands".
§ A ligand is an ion or molecule that bonds to a metal ion.
§ Ligands are Lewis bases; this means that a ligand must have
a lone pair of electrons to donate to a metal centre.
§ Conversely, the metal acts as a Lewis acid; it accepts the
electrons.
§ The donation of a lone pair of electrons in this manner
results in a "coordinate bond".
If provided with a ligand, you will need to be able to determine the charge and the
binding sites, and how many atoms it can bind to a single metal ion (e.g. if it is
monodentate or bidentate).
Monodentate ligands
- The table to the right shows examples of monodentate ligands.
- Some ligands are anionic, while others are neutral.
- Also, notice that for all of these ligands, coordination to the metal
occurs via a single donor atom.
As such, these ligands are all "monodentate" ligands.
Multidentate ligands
Ligands that can form more than one coordinate bond are said to be
"multidentate" (bidentate, tridentate, tetradentate, etc.).
- Once again, the ligands may be either anionic or neutral.
, - Complexes containing multidentate ligands are called "chelate" complexes.
Counting d-electrons in coordination compounds
Consider the coordination compound shown to the right.
- You’ve already seen how to count the number of d-electrons in an
"isolated" transition metal ion, but how do you count the d-electrons in a
coordination compound like this?
The process is similar to an "isolated" transition metal ion; though now you
must also account for any charges on the ligands.
1. Determine whether the ligands are neutral or anionic.
- Imagine that a ligand departed from the coordination compound with the bonding electrons.
§ Would the resulting free ligand be neutral or charged?
- In the particular example given above, [CoCl4]2-, the ligands are all anionic (charge = –1).
2. Determine the oxidation state of the metal.
- [CoCl4]2- has an overall charge of –2.
§ The total charge of the ligands is 4 x (–1) = –4.
- Therefore, to get the overall charge right, the transition metal must have a charge (i.e. an oxidation state) of
+2.
3. Determine the d-electron count.
- Using the arrows-in-boxes method (earlier in the lesson), you can figure out that Co2+ has 7 d-electrons.
§ We can find this as we now know the oxidation state of the metal.
Coordination number and complex geometries
The coordination number is defined as the number of donor atoms bonded to a metal centre.
- We consider the geometries of coordination compounds below.
CHEM1021
,CHEM1021
Bold (blue) – Week number and core focus
Italic (blue) – Pre-lecture or lecture content
Italic and underlined (black) – Heading
Bold (black) – Subheading
Week 1: Inorganic Chemistry I – Transition Metal Complexes
Pre-lecture content
Transition metals and their d-electron counts
The definition of a transition metal is ‘an element that has a partially filled d sub-shell’.
- The transition metals (with variable oxidation state usually +2 or +3) therefore include:
- Remember that a filled or a half-filled set of 3d orbitals is energetically favourable, leading to
anomalous behaviour for Cr and
Cu.
Counting d-electrons in transition metals ions
- Transition metals is that they can
exist in a variety of oxidation
states (Fe2+, Fe3+).
- Different oxidation states have
different d-electron counts.
§ This is important because
the d-electron count
, affects many chemical properties.
- A key point to remember is that in transition metal ions, the 3d orbitals are lower in energy than the 4s orbital
(the reverse of the situation with neutral transition metal atoms).
§ Hence, in a transition metal ion, the 4s orbital will usually be empty.
- The d-orbital electrons in a transition metal ion is group number (1-18) minus oxidation state (charge).
Coordination compounds
In reality, transition metal ions are rarely isolated as single atoms floating in space.
- Usually, transition metal ions are surrounded by, and bound to, a group of small molecules giving a
"coordination compound".
Let’s examine the following example:
- Consider an Fe3+ ion in aqueous solution.
- The positive charge of the Fe3+ ion will interact with the lone pairs of electrons on water molecules.
§ Bonds will form!
- In this example, the water molecules are said to be "ligands".
§ A ligand is an ion or molecule that bonds to a metal ion.
§ Ligands are Lewis bases; this means that a ligand must have
a lone pair of electrons to donate to a metal centre.
§ Conversely, the metal acts as a Lewis acid; it accepts the
electrons.
§ The donation of a lone pair of electrons in this manner
results in a "coordinate bond".
If provided with a ligand, you will need to be able to determine the charge and the
binding sites, and how many atoms it can bind to a single metal ion (e.g. if it is
monodentate or bidentate).
Monodentate ligands
- The table to the right shows examples of monodentate ligands.
- Some ligands are anionic, while others are neutral.
- Also, notice that for all of these ligands, coordination to the metal
occurs via a single donor atom.
As such, these ligands are all "monodentate" ligands.
Multidentate ligands
Ligands that can form more than one coordinate bond are said to be
"multidentate" (bidentate, tridentate, tetradentate, etc.).
- Once again, the ligands may be either anionic or neutral.
, - Complexes containing multidentate ligands are called "chelate" complexes.
Counting d-electrons in coordination compounds
Consider the coordination compound shown to the right.
- You’ve already seen how to count the number of d-electrons in an
"isolated" transition metal ion, but how do you count the d-electrons in a
coordination compound like this?
The process is similar to an "isolated" transition metal ion; though now you
must also account for any charges on the ligands.
1. Determine whether the ligands are neutral or anionic.
- Imagine that a ligand departed from the coordination compound with the bonding electrons.
§ Would the resulting free ligand be neutral or charged?
- In the particular example given above, [CoCl4]2-, the ligands are all anionic (charge = –1).
2. Determine the oxidation state of the metal.
- [CoCl4]2- has an overall charge of –2.
§ The total charge of the ligands is 4 x (–1) = –4.
- Therefore, to get the overall charge right, the transition metal must have a charge (i.e. an oxidation state) of
+2.
3. Determine the d-electron count.
- Using the arrows-in-boxes method (earlier in the lesson), you can figure out that Co2+ has 7 d-electrons.
§ We can find this as we now know the oxidation state of the metal.
Coordination number and complex geometries
The coordination number is defined as the number of donor atoms bonded to a metal centre.
- We consider the geometries of coordination compounds below.
