CHEM1021 – A/Prof Graham Ball – Lecture 2
Ligands & Complexes
2
Learning Outcomes
By the end of today’s lecture you should know
e able to do the following:
1. Explain what is meant by a ligand
2. Predict the denticity of a ligand and identify the donor atoms
3. Assign coordination numbers to complexes
4. Be able predict the geometry of a complex given its CN (2-6)
5. Be able to name transition metal complexes
Transition Metal (TM) ions in water:
coordination compounds/ complexes
TM ions “su
ounded” by water molecules
to make a complex
Attached molecules are ligands
Note how the formula is writtten:
[Co(OH2)6]
2+
- Metal first
- ligands in
ackets (in alphabetical order)
- donor atoms first (this rule is frequently ignored)
- charge on the complex outside square
ackets
Transition metal complexes: formulae
• The complex formula is written between square
ackets (the metal ion
first, followed by the ligands in alphabetical order)
• ions outside the square
ackets are termed counterions
• Eg : Na3[FeCl6] e.g. [Cu(NH3)6]SO4
- cations first, 3 x Na+ - cation first [Cu(NH3)6]
2+
- anion is [FeCl6]
3- - anion is SO4
2-
History of coordination compounds
Transition metal complexes are often called coordination compounds
History of coordination compounds
What’s my name?
What do you notice about the compounds in green boxes?
What’s unusual about
this compound?
7
Complexes I
• You have actually encountered complexes before – [Al(H2O)6]
3+
• The majority of complexes (sometimes called coordination compounds)
contain transition metals
• They have a variety of uses and applications (vide infra)
• Tasseart made a series of CoCl3(NH3)n compounds (where n = 4, 5, 6)
in the 1790’s
• Werner (in 1890) described these as Co coordinated by 6 ‘Ligands’
• Number of ligands around the central metal ion can vary
Alfred Werner – pioneer of coordination chemistry
• XXXXXXXXXXfirst to propose
co
ect structures for
coordination compounds -
central transition metal
atom is su
ounded by
ligands
• Noble prize 1913
9
Complexes II
• A compound is said to be a ‘complex’ when the coordination numbe
(number of donor atoms/coordinate bonds) is larger than the oxidation
state
no of Donor atoms > Oxidation State
e.g. [Cu(NH3)4(H2O)2]
2+
• We use [square]
ackets around a complex
• Ligands can be neutral or negatively charged
• Transition metal complexes can be positively or negatively charged or
neutral
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Ligands
• A ligand is an ion or molecule that binds to a metal ion
• Ligands are Lewis bases (donate electrons), this means that they have a
lone pair of e- to donate to a metal centre
• The atom on which the lone pair is located is called the donor atom.
• The donation of a lone pair of e- results in a ‘coordinate bond’
• Ligands almost exclusively neutral or negatively charged
Denticity…
• As we can see, ligands may have one or
more donor atoms.
• Those with one donor are termed
monodentate (meaning one ‘tooth’) and
can only occupy one coordination site.
• Examples: HO-, NH3, OH2, Cl
-, I-, CO, NC-
http:
media.photobucket.com/use
MustardColonel/media/Conrad_Poohs.png.html?filters[term]=conrad%20poohs&filters[p
imary]=images
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Types of Ligands
• Ligands that only form one coordinate bond with a metal centre (take up
one ‘coordination site’) are said to be monodentate (or unidentate)
• The word dentis in Latin means tooth, so monodentate means ‘one-toothed’
• Ligands that take up more than one coordination site are said to be
multidentate (bidentate, tridentate, tetradentate etc.)
• Complexes of multidentate ligands are called ‘chelate complexes’
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Monodentate Ligands
• Below is a list of common monodentate ligands; note the nomenclature
used – most commonly ions that end in ‘e’ become ‘o’ when in a complex
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Bidentate Ligands
• Below are some common bidentate ligands, remember these multidentate
ligands form chelate complexes (chēlē means claw in Greek)
tris(oxalato)fe
ate(III) trianion
O
O O
O
2–
oxalate anion
(ox2–)
Complexes of bidentate Ligands
O O
–
acetylacetonate anion
(acac–)
tris(acetylacetonato)chromium(III)
N N
1,10-phenanthroline
(phen)
dichlorobis(phenanthroline)ruthenium(II)
H2N NH2
1,2-diaminoethane
(ethylenediamine; en)
dichlorobis(ethylenediamine)cobalt(III)
cation
16
Multidentate Ligands
• Below are some multidentate ligands
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Try it for yourself – Identify denticity
What would the denticity of this ligand be…
1. 1
2. 2
3. 3
4. 4
5. 5
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Try it for yourself – Identify denticity
What would the denticity of this ligand be…
1. 1
2. 2
3. 3
4. 4
5. 5
F-
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Try it for yourself – Identify denticity
What would the denticity of this ligand be…
1. 1
2. 2
3. 3
4. 4
5. 5
20
Try it for yourself – Identify denticity
What would the denticity of this ligand be…
1. 1
2. 2
3. 3
4. 4
5. 5
21
Try it for yourself – Identify denticity
What would the denticity of this ligand be…
1. 1
2. 2
3. 3
4. 4
5. 5
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Haemoglobin
Heme
23
Try it for yourself – Identify denticity
What would the denticity of this ligand be…
1. 2
2. 3
3. 4
4. 5
5. Not sure/I have a different numbe
24
EDTA
• Ethylenediaminetetraacetic acid (or H4edta/edta/EDTA) is a multidentate
ligand that is capable of forming six coordinate bonds with a metal ion
• It is often used in ‘chelation therapy’ to combat metal overload disorder (can
ind metals into a complex that can be ‘removed’ from the body via the
kidneys)
= Donor atoms
Binds as EDTA4-
Transition metal complexes – overall charge and
oxidation numbe
The overall charge on the complex is the sum of the charges on the metal
ion and ligands:
e.g. Cu XXXXXXXXXXEDTA4- → [Cu(EDTA)]2-
• Therefore:
Oxidation state of transition metal ion = charge on the complex
– sum of charges on the ligands
Example : For [FeCl6]
3-: Oxidation state of Fe = (-3) – (-1 x 6) = 3+
Try it for yourself
What is the oxidation number of the metal in this
complex?:
1) +1
2) -1
3) +2
4) -2
5) +3
What is the d electron count of the metal in this
complex:
1) 3
2) 4
3) 5
4) 6
5) 7
Try it for yourself
What is the oxidation number of the metal in [PtCl6]
2- :
1) +1
2) +2
3) +3
4) +4
5) -2
Try it for yourself
What is the d electron count of the metal in [PtCl6]
2- :
1) 4
2) 5
3) 6
4) 7
5) 8
Try it for yourself
30
Coordination Number and Geometries of Complexes
• Transition metal complexes exhibit a huge variety of structures
• The coordination number (CN) is defined as the number of donor atoms
directly attached to the metal ion
• Certain geometries are associated with each coordination numbe
• You are expected to know the potential shapes of complexes with CN of 2-6
• Can get a little more complicated than that (some ligands will be ‘formally 3
coordinate’ etc.)
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Coordination Number of 2
• CN = 2
• These are rare – CuI, AgI, AuI and HgII
• They form linear complexes
Coordination
Geometry:
32
Coordination Number of 3
• CN = 3
• These are also rare – CuI, and HgII (you should assume a CN of 2 unless
explicitly stated in molecular formula)
• They form trigonal planar complexes
Coordination
Geometry:
33
Coordination Number of 4
• CN = 4
• There are two common shapes that form, tetrahedral and square plana
• Complexes with halides (or d10 ions) tend to form tetrahedral complexes,
where as d8 ions (like RhI, IrI, PdII, PtII, AuIII and NiII) tend to form square
planar complexes
Coordination
Geometry:
34
Coordination Number of 5
• CN = 5
• There are two common shapes that form, square pyramidal and trigonal
ipyramidal
• These are rare, and in reality are often distorted shapes, resulting from
metal ligand interactions
Coordination
Geometry:
35
Coordination Number of 6
• CN = 6
• There are two shapes that form, octahedral (the most common coordination
geometry) and trigonal prismatic (extremely rare – need ‘special’
a
angement of ligands)
Coordination
Geometry:
36
Higher Coordination Numbers
37
Try it for yourself
What is the CN of the metal ion in the molecule pictured?
1. 2
2. 3
3. 4
4. 5
5. 6
38
Try it for yourself
What is the CN of the metal ion in the molecule pictured?
1. 2
2. 3
3. 4
4. 5
5. 6
39
Try it for yourself
What is the CN of the metal ion in the molecule pictured?
1. 2
2. 3
3. 4
4. 5
5. 6
40
Try it for yourself
What geometry would you expect [V(H2O)6]
3+ to exhibit?
1. Square Plana
2. Trigonal Plana
3. Tetrahedral
4. Octahedral
5. Trigonal Bipyramidal
41
Try it for yourself
What geometry would you expect [FeCl4]
- to exhibit?
1. Square Plana
2. Trigonal Plana
3. Tetrahedral
4. Octahedral
5. Trigonal Bipyramidal
42
Try it for yourself
What geometry would you expect [Pt(NH3)2Cl2] to exhibit?
1. Square Plana
2. Trigonal Plana
3. Tetrahedral
4. Octahedral
5. Trigonal Bipyramidal
43
Try it for yourself – Think Critically…!
What geometry would you expect [Fe(bpy)3]
2+ to exhibit?
1. Square Plana
2. Trigonal Plana
3. Tetrahedral
4. Octahedral
5. Trigonal Bipyramidal
44
Try again…
What geometry would you expect [Fe(bpy)3]
2+ to exhibit?
1. Square Plana
2. Trigonal Plana
3. Tetrahedral
4. Octahedral
5. Trigonal Bipyramidal
45
η Vs. μ Ligands
• One final bit of ligand nomenclature…
• η (eta) can be placed in front of a ligand to denote how many coordinate
onds are formed to an uninte
upted and contiguous series of atoms e.g.
the benzene rings below are said to be η6
• Although this may seem a
itrary, some multidentate ligands may only form
one coordinate bond with a metal – beyond the scope of this course
• μ (mu) can be used to say that a ligand
idges more than one metal centre
e.g. a SCN ligand may be able to form coordinate bonds with two different
metal centres, we would say this is μ2
η6-benzene
µ-Cl
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IUPAC Rules for Naming Complexes
• For ionic compounds, cation named before anion
• When naming complexes (complex ions) ligands named first
(alphabetically), metal atom/ion named last (with oxidation state is given in
(Roman or Arabic) numerals in parentheses)
• Names of anionic ligands end with suffix ‘…o’ change …ide to …ido,
change …ite to …ito, change …ate to …ato
• Neutral ligands retain their usual name (e.g. ethylenediamine), exceptions:
aqua, ammine, ca
onyl, nitrosyl
• Number of each type of ligand is indicated by Greek prefix: di… 2; tri… 3;
tetra… 4; penta… 5; hexa… 6
• If the ligand contains a Greek prefix, indicate number of ligands with: bis…
2; tris… 3; tetrakis… 4; pentakis… 5; hexakis… 6
• Name for a complex anion ends in …ate with Latin stem (if none, English
stem) e.g