d-orbital occupation and electronic configurations
To be able to use Crystal Field Theory (CFT) successfully, it is
essential that you can determine the electronic
configuration of the central metal ion in any complex.
This requires being able to recognise all the entities making up
the complex and knowing whether the ligands are neutral or
anionic, so that you can determine the oxidation
state of the metal ion.
In many cases the oxidation state for first row transition metal
ions will be either (II) or (III), but in any case you may find
it easier to start with the M(II) from which you can easily add
or subtract electrons to get the final electronic configuration.
A simple procedure exists for the M(II) case.
First write out all the first row transition metals with their
symbols and atomic numbers:
| 22 |
23 |
24 |
25 |
26 |
27 |
28 |
29 |
| Ti |
V |
Cr |
Mn |
Fe |
Co |
Ni |
Cu |
To see the number of electrons in the 3d orbitals then cross
off the first 2, hence:
So, the electronic configuration of Ni(II) is
d8 and the electronic configuration of
Mn(II) is d5.
What is the electronic configuration of Fe(III)?
Well, using the above scheme, Fe(II) would be d6, by
subtracting a further electron to make the ion more positive, the
configuration of Fe(III) will be
d5.
This simple procedure works fine for first row transition
metal ions, but sorry it is no good for 2nd or 3rd row
elements!
Note: For all final Chemistry examinations, a Periodic Table
is provided in the inside back cover of the examination booklets.
A Periodic Table may NOT necessarily be provided for course
tests.
Oxidation States and their Relative Stabilities
The transition metals show a wide range of oxidation states.
The reason for this is the closeness of 3d and 4s energy states as discussed
above. The Table below summarises known oxidation numbers of the
first row transition elements. The most prevalent oxidation
numbers are shown in bold and those in blue are likely to be met in this course.
Known Oxidation Numbers of First Row Transition Elements*
| Sc |
|
|
+3 |
| Ti |
+1 |
+2 |
+3 |
+4 |
| V |
+1 |
+2 |
+3 |
+4 |
+5 |
| Cr |
+1 |
+2 |
+3 |
+4 |
+5 |
+6 |
| Mn |
+1 |
+2 |
+3 |
+4 |
+5 |
+6 |
+7 |
| Fe |
+1 |
+2 |
+3 |
+4 |
+5 |
+6 |
| Co |
+1 |
+2 |
+3 |
+4 |
+5 |
| Ni |
+1 |
+2 |
+3 |
+4 |
| Cu |
+1 |
+2 |
+3 |
| Zn |
|
+2 |
* The oxidation number zero usually assigned to the elemental state
has been omitted from the Table. The elements Cr to Co form
several metal carbonyl compounds where the metals are considered to be in
oxidation state zero.
A number of important conclusions can be drawn from this Table.
1. There is an increase in the number of oxidation states from Sc
to Mn. All seven oxidation states are exhibited by Mn. The
oxidation number of +7 represents the formal loss of all seven
electrons from 3d and 4s orbitals. In fact all of the elements in
the series can utilize all the electrons in their 3d and 4s
orbitals.
2. There is a decrease in the number of oxidation states from Mn
to Zn.
This is because the pairing of d-electrons occurs after Mn
(Hund's rule) which in turn decreases the number of available
unpaired electrons and hence, the number of oxidation states.
3. The stability of higher oxidation states decreases in moving
from Sc to Zn. Mn(VII) and Fe(VI) are powerful oxidizing agents
and the higher oxidation states of Co, Ni and Zn are unknown.
4. The relative stability of the +2 state with respect to higher
oxidation states, particularly the +3 state increases in moving from
left to right. This is justifiable since it will be increasingly
difficult to remove the third electron from the d orbitals.
5. There is a tendency of intermediate oxidation states to
disproportionate. For example,
Mn(VI) → Mn(IV) + Mn(VII)
Cu(I) → Cu(0) + Cu(II).
6. The lower oxidation states are usually found in ionic
compounds and higher oxidation states tend to be involved in
covalent compounds.
The relative stability of oxidation states is an extremely
important topic in transition metal chemistry and is usually
discussed in terms of the standard reduction potential (E°)
values. Thermodynamically E° values are equated to
ΔG° values in the form of the well known relationship:
ΔG° = -nFE° where n = number of electrons involved
and F = Faraday of electricity. Hence, the E° values indicate
the possibility of spontaneous change from one oxidation state to
the other. This value however, does not give any information
about the reaction rate. Predictions regarding the stability of a
particular oxidation state of an element can be made from the
Tables of Redox values found in any standard text book or Data
Book. The approach can be illustrated from the example below;
return to C10K course outline
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Lancashire,
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Created June 1997. Last modified 2nd February 2010.
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