Download Theory of Coordination Chemistry

2P32 – Principles of Inorganic Chemistry
Dr. M. Pilkington
Lecture 3 - Classification and Nomenclature
1.
Ligand Classification:
 Coordination Chemistry and Ligands
 Monodentate Ligands
 Ambidentate Ligands
 Bridging Ligands – Biological Applications
 Multi/Polydentate Chelating Ligands
2.
Naming Metal Complexes
Rodgers Chapter 2.
Theory of Coordination Chemistry
Alfred Werner (1866-1919)

1893, age 26: coordination theory

Nobel prize for Chemistry, 1913

Addition of 6 mol NH3 to CoCl3(aq)
Coordination compound/complex.
N
H
3+
NH3
H
M
H
N forms a coordinate
covalent bond to the
metal
H3N
H3N
Co
NH3
NH3
3Cl–
(counter ion)
NH3
ligand (coordination sphere)
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1. Ligand Classification
Ligand – Lewis base (electron pair donor) that is bonded
to a metal ion. Ligands are anionic or neutral.
H
]3+
i.e. [Fe(NH3)6
Metal Complex
Fe
N
H
H
Ligand - Lewis Base
Has one pair of electrons
:NH3
The NH3 shares its electron with the Fe(III) metal ion.
Classes of Ligands
1. Monodentate Ligands
“one toothed” – bind to a metal ion through a single
donor site.

For example :NH3 is a monodentate ligand.
Co3+
:NH3
Fe3+
Cl
Fe2+
C
N
2
2. Bridging Ligands
Bind to two or more metal ions simultaneously.

For Example:
O2Cl
Fe
C
N
2
(H2O)5Fe
Co3+
O
Cl
neutral ligand
Fe(H2O)5
Co3+
n+
5+
Fe
[(NC)5Fe(III)CNFe(III)(CN)5]5-
Fe can exist in number of oxidation states.
A biologically important metal ion.
A Biological Application of Fe
Iron-sulfur proteins are proteins characterized by the presence
of iron-sulfur clusters containing sulfide-linked iron centers in
variable oxidation states.
Structural motifs
 In almost all Fe-S proteins, the Fe centers is tetrahedral and
the thiolato sulfur centers, from cysteinyl residues, are terminal
ligands. The sulfide groups are either two- or threecoordinated. A common motif features a four iron ions and four
sulfide ions placed at the vertices of a cubane-type structure.
4Fe-4S clusters
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Aconitase - aconitate hydratase; is an enzyme that catalyses the stereospecific isomerization of citrate to isocitrate via cis-aconitate in the
tricarboxylic acid cycle, a non-redox-active process.
Illustration of pig aconitase in complex with the [Fe4S4] cluster. The
protein is colored by secondary structure, and iron atoms are blue and the
sulfur red.
3. Ambidentate Ligands
Two kinds of binding sites – the ligand can bind one metal ion
through one or the other but not both simultaneously.
:C
Ambidentate
N:
Fe2+ CN It is not possible for N to bind to the same Fe2+
NO2-
O
N
O
lone pairs on O and N
can bind through either lone pair
:N
C
S
Thiocyanide ligand can bind through S or N but not both at the
same time to one metal ion.
You will need to be able to draw the Lewis acid structures correctly so you
can figure out how a ligand will bind.
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4. Multi-/Polydentate Chelating Ligands
Multidentate – “multitooth”; chelating - “crab” claw
Ligands that are bound to a metal through several donor sites.
Chelate – “Chelos” (greek) meaning “crab” (crabs grab their food
with two claws, in the same way a metal can be attracted by two
lone pairs from different groups on the same ligand).
Example 1. Ethylenediamine H2NCH2CH2NH2 (en)
Forms a 5-membered chelate ring; you can think of it havings “2
claws” coming in to grab the metal.

3
H2C
H 2
N
H
4
CH2
N
H
5 H
M
1
bidentate "two-toothed"
5-membered chelate ring
The Chelate Effect

For a given metal ion, the thermodynamic stability of a chelated
complex involving bidentate or polydentate ligands is greater
than that of a complex containing a corresponding number of
comparable monodentate ligands.

This is called the chelate effect.

5-membered rings are more stable than 6,

4-membered rings (or smaller) are not stable due to steric
strain.
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Example 2. Ethylenediaminetetraacetate (ETDA)
O
O
O
C
C
CH2
CH2
O
O
M
NCH2CH2N
O
C
CH2
O
N
CH2
O
C
O
N
O
O
O
[M(EDTA)]n-
two Nitrogen's and four Oxygens bond to
a single metal ion - Hexadentate Ligand.
When we draw the ligand chelated to the metal ion we
do not draw all of the carbons.
-
O
N
Cr
O
N
Cr
O
O
O
N
[Cr(edta)]–
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Examples of Multidentate Chelating Ligands

Example 3. -Diketones


R C
O
2,4-pentadione, Ketone group on  carbon
Ketone form
CH2 C R'
diketo O
equimolar with another form
R
O
R
Enol form
the alcohol has an OH ending
CH C R'
C
enol
OH
C
CH C R'
O
O
the enol form can easily dissociate, i.e.
loose a H
enolate anion


-Diketones – bidentate ligands
e.g. R= CH3 Acetylacetone
R
C
H
C
R'
R
C
O
O
H
C
O
O
M
R'
M
where R = CH3
acetylacetone
R'
R
Chelate Ring
O
O
M
delocalized electrons
in the double bond
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Make sure you can draw the structures and metal complexes
of all of the ligands on your ligand sheet.
Problem – Test 2007 worth 8 marks
1. Draw Lewis structures for the following ligands:
(i) Pyridine C5H5N (2 marks)
(ii) Nitrato NO3(2 marks)
(iii) Nitro
NO2- (2 marks)
Which ligand(s) above is/are ambidentate and why?
(2 marks)
2. Naming Metal Complexes – refer to handout and Rodgers
textbook (pg 20-26) for rules and examples.
1.
For complex ions write the cation first and anion last e.g. K2[PtCl4] –
Potassiumtetrachloroplatinate.
(note you do not use the prefix mono, di or tri etc.. here to
indicate the number of cations)
2.
Name the ligands first in alphabetical order, the metals last.
3.
Prefixes to indicate numbers. (di, tri, tetra, penta, hexa etc…)
for all monoatomic ligands, polyatomic ligands with short names and
neutral ligands with special names. (see Table 2.4 Rodgers).
4.
Prefixes bis-, tris-, tetrakis-, pentakis-, hexakis- for ligands whose
names contain a prefix of the first type, neutral ligands without
special names, ionic ligands with particularly long names.
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For example:
[Cr(H2O)4Cl2]+ - tetraaquodichlorochromium(III) ion.
[Cr(NH2CH2CH2NH2)3]3+ - tris(ethylenediammine)chromium(III) ion.
Note the parenthesis around the organic ligand name.
5.
If the anion is complex, add the suffix –ate to the name of the metal.
If the symbol comes from latin/greek, then we go back to the
latin/greek for the name of the anion.
6.
Put the oxidation state in Roman numerals in parantheses after the
name of the central metal ion.
7.
Practice to get the hang of this – Examples from Rodgers and Practice
Handout.
For example:
[CoCl4]2- - tetrachlorocobaltate(II) ion
[Fe(CN)6]4- - hexacyanoferrate(II) ion
Ligand Names
Neutral Ligands - they have the same name as the molecule with
4 exceptions.
Molecule Name
Ligand Name
NH3
Ammonia
Ammine
H2O
Water
Aquo
CO
Carbon monoxide Carbonyl
NO
Nitric oxide
Nitrosyl


Anionic Ligands (end in –ide or –e) – add –o
Ion
Ligand Name
ClChloride
Chloro
OH
Hydroxide
Hydroxo
2SO4
Sulfate
Sulfato
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Naming Metal Complexes
Fe
Cu
Ag
Au
Sn
Pb
Hg
Greek/Latin Ligand
ferrum
ferrate
cuprum
cuprate
argentum
argentate
aurum
aurate
stannum
stannate
plumbum
plumbate
hydrorgyrum mercurate
Bridging Ligands – use  to indicate a bridge.
 If there is more than one of a given bridging ligand, the prefix
indicating the number of ligands is placed after the .
 If there is more than one different bridging ligands, they are
given in alphabetical order.
For example:
O22- peroxide
O
O
(H3N)4CoIII
3+
CoIII(NH3)4
NH2
amide
Tetraamminecobalt(III)--amido--peroxo-tetraamminecobalt(III) ion
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Naming Metal Complexes
Examples:

[Co(NH3)6]3+ hexaamminecobalt(III) ion.

[PtCl2(NH3)2] diamminedichloroplatinum(II)

[Fe(CN)6]4- hexacyanoferrate(II) ion.

[Fe(H2O)6]2+ hexaaquoiron(II) ion.
Problem - Test 2007
Name the compounds:
1. Na[Co(H2NCH2CH2NH2)(C2O4)ClBr]
marks)
2. [Zn(EDTA)]2-
(2
(2 marks)
Pay particular attention to the naming of the ambidentate
ligands
We distinguish which atom is bound to the metal in the naming
e.g. SCN- is named as thiocyanato, but NCS- is named as
isothiocyanato
CN- - cyano, but NC- - isocyano
NO2- - nitro, but ONO- is nitrito
Rodgers Chapter 2, page 22
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