Download Coordination compounds - Structure

NPTEL – Chemistry and Biochemistry – Coordination Chemistry (Chemistry of transition
elements)
Coordination compounds - Structure
K.Sridharan
Dean
School of Chemical & Biotechnology
SASTRA University
Thanjavur – 613 401
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Table of Contents
1 Different types of ligands ............................................................................................................. 3
1.1 Ligand .................................................................................................................................... 3
1.1.1 Types of ligands .............................................................................................................. 3
2 Complexes of different types of ligands ....................................................................................... 5
2.1 Complexes with Monodentate ligands.................................................................................. 5
2.2 Bidentate ligands - Chelates .................................................................................................. 5
2.3 Tridentate chelates................................................................................................................ 6
2.4 Tetradentate ligands ............................................................................................................. 6
2.5 Hexadentate ligand ............................................................................................................... 6
2.6 Detection of complex formation ........................................................................................... 7
2.7 How to confirm complex formation? .................................................................................... 7
3. Coordination geometry ............................................................................................................... 8
3.1 Coordination number 2 ......................................................................................................... 8
3.2 Coordination number 3 ......................................................................................................... 8
3.3 Coordination number 4 ......................................................................................................... 9
3.3.1 Tetrahedral complexes ................................................................................................... 9
3.3.2 Square planar complexes ............................................................................................... 9
3.4 Coordination number 5 ....................................................................................................... 10
3.5 Coordination number 6 ....................................................................................................... 10
4 Nomenclature of coordination compounds ............................................................................... 10
4.1Symbol of complexes............................................................................................................ 10
4.2 Names of complexes ........................................................................................................... 11
4.3 Oxidation state .................................................................................................................... 11
4.4 Isomers ................................................................................................................................ 11
4.5 Special ending...................................................................................................................... 11
4.6 Prefixes for number of ligands ............................................................................................ 11
4.7 Vowels in the name of ligands............................................................................................. 12
4.8 Ligand name ending ............................................................................................................ 12
5 References .................................................................................................................................. 14
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1 Different types of ligands
1.1 Ligand
Any species capable of donating a pair of electrons to a metal is called a ligand.
It may be an ion, negatively or positively charged, or a neutral molecule.
Examples: Cl-, Br-, SO 4 2-, NH2NH 3 +, NH 3 , H 2 O, NH 2 CH 2 CH 2 NH 2 etc.
1.1.1 Types of ligands
Ligands can be classified as monodentate, bidentate, polydentate etc. ligands.
1.1.1.1 Monodentate ligand
This will coordinate to only site of a metal ion. In other words, it can donate only
one pair of electrons to the metal ion.
Example: Cl-, Br-, SO42-, NH2NH3+, NH3, H2O
1.1.1.2 Bidentate ligand
This will occupy two sites of a metal ion. That is, it can attach itself to two
positions of a metal ion.
Example: NH2CH2CH2NH2 etc
1.1.1.3 Polydentate ligands
These ligands occupy many sites of the same metal ion. Example: EDTA etc.
Table 1.1 lists some of the ligands belonging to different types
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Table 1.1 Ligands, names, structures and denticity
Name
Formula
Ethylenediamine
NH2CH2CH2NH2
Denticity
1,10-Phenanthroline
2
en
2
phen
2
diars
2
diphos
2
acac
N
N
o-Phenylenebis(di
methylarsine)
Abbreviation
AsMe2
AsMe2
1,2-Bis(diphenyl
phosphino)ethane
acetylacetonate
CH2
P(C6H5)2
CH2
P(C6H5)2
CH3COCH=CCH3
O
H
C
salicylaldimine
NR
2
sal
2
oxinate
2
ox
3
terpy
O
8-Quinolinate
N
O
oxalate
O
C
C
O
O
O
terpyridine
N
N
N
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diethylenetriamine
CH2
CH2
NH2
CH2
CH2
NH2
HN
CH2
triethylenetetramine
N
CH2
3
dien
4
tren
NH2
NH2
CH2
CH2
NH2
CH2
CH2
ethylenediaminetetraacetate
O
O
O
C
CH2
H2C
N
O
C
H2
C
CH2
O
N
CH2
H2C
C
O
O
6
C
O
EDTA
2 Complexes of different types of ligands
2.1 Complexes with Monodentate ligands
NH3 ligands: Zn(NH3)42+, Ni(NH3)62+, Co(NH3)63+, Fe(H2O)6, Cr(CO)6 etc.
2.2 Bidentate ligands - Chelates
When ligands other than monodentate ligands bind to a metal ion, a ring complex
is formed. This is called a chelate. Example: ethylenediamine-metal complex
CH2
CH2
NH2
NH2
Co
N
N
Cr
metal-en chelate
Metal-bipyridyl
chelate
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2.3 Tridentate chelates
NH2
Fe
NH
NH2
diethylenetriamine
complex
2.4 Tetradentate ligands
NH3
H2N
Fe
N
NH2
triethylenetetramine
complex
2.5 Hexadentate ligand
O
O
O
N
O
Fe
O
N
O
O
O
Fe-EDTA complex
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2.6 Detection of complex formation
1. Most of the complexes will be colored except d5(weak-field) and d10
systems. Hence, when a solution of the ligand is added to a solution of a
metal salt, there will be a change in the color of metal salt solution. For
example, when ammonia is added in excess to aqueous copper sulfate
solution (blue), the color changes to deep blue indicating complex
formation. This is the simplest way of detecting complex formation.
2.
UV-visible spectrum can be used to further support this. The max of
the ligand (if already absorbs) and the metal salt solution (if already
colored) will be shifted.
3.
When the complex is decomposed with concentrated nitric acid, the
original color of the metal salt solution will be restored.
4. The original property of the metal ion will be lost during complex
formation.
For example, aqueous solution of Fe3+
will give brown
color with KCNS(aq). However, a complex of Fe3+ will not answer this
test.
2.7 How to confirm complex formation?
1. Elemental analyses – C, H, N, S analyses
The percentages of these elements are determined by Elemental analyzer
and matched with the calculated value for the proposed structure. If these
two match, then the proposed structure of the complex is correct.
2. Molar conductance measurements will tell us whether the complex is neutral
or charged and also will tell whether it is a 1:1, or 1:2 etc complex.
3. Magnetic susceptibility measurements will tell us the number of unpaired
electrons in the complex, if it is paramagnetic, and hence, the oxidation state of
the central metal ion.
4. The coordination of the ligand with the metal is confirmed by IR and far IR
spectra. The IR spectrum of the ligand is compared with that of the complex and
from the shift in values of the N-H, C=O, O-H etc. we can confirm which atom is
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coordinated to the metal ion.
5. Electronic spectrum (UV-vis) of the complex will give us the oxidation
state of the central metal and the geometry of the complex.
6. EPR spectrum for a paramagnetic complex through its hyperfine splitting will
give us the number of unpaired electrons in the central metal, ‘g’ value will
give us information about the distortion.
7. TG and DTA will tell us about the thermal stability of the complex and also
about the decomposition of the ligands.
From all these, we confirm the structure of the complex.
3. Coordination geometry
3.1 Coordination number 2
Examples: Cu(NH3)2+, AgCl2-, Au(CN)2-, HgCl2
These are linear and the ligand-metal-ligand bond angel is 1800; Cl – Ag – ClThese metal ions have d10 configuration in the ground state.
Even though the metal–ligand bond may be considered to be formed due to
the overlap of a σ-orbital of the ligands with sp-hybridized metal orbital, actually
some d-orbital contribution is also there in the bond formation. These hybrid
orbitals will have 50% pz character along with a small amount of dz2 character
and the remaining will be ‘s’ character.
3.2 Coordination number 3
It is a rare coordination number. In many crystalline compounds, the
stoichiometry may be MX3 but the actual coordination number may be greater
than 3. When the ligands are extremely bulky, this coordination number exists.
In some of the d10 systems, this coordination number exists, even when the
ligands are not bulky. Examples: KCu(CN)2, Pt(PPh3)3 etc. In such complexes,
the metal atom and the ligands directly connected to it lie on the same plane.
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3.3 Coordination number 4
This is one of the most common and important coordination numbers. Two
geometries are there for this coordination number, viz., tetrahedral and square
planar.
3.3.1 Tetrahedral complexes
If the central metal atom is not having any lone pair of electrons and if it
is not a transition metal, then it will form a tetrahedral complex.
Examples: BeCl 4 2-, BF 4 -, SnCl 4 , AlF 4 - etc.
Cl
Sn
Cl
Cl
Cl
On the other hand, if the central metal atom is a transition metal but it
does not have a d8 configuration, then it forms a tetrahedral complex.
Example: FeCl 4 -, CoCl 4 2-, MnO 4 - etc.
If the central metal atom has d8 configuration, usually it will form a square
planar complex. However, it forms tetrahedral complexes sometimes.
Example: NiCl 4 2-, NiCl 3 OPPh 3 . When the ligands are bulky, Co(I) and Ni(II)
form tetrahedral complexes.
3.3.2 Square planar complexes
If the central metal ion has a d8 configuration, mostly it will form square
planar complexes.
Example: Ni(CN)42-, PdCl42-, PtCl42- etc.
Cl
Cl
Pt
Cl
Cl
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3.4 Coordination number 5
Two geometries are possible for this coordination number, viz., t r i g o n a l
bipyramidal and square pyramidal.
M
M
Square planar
Trigonal bipyramid
Square pyramidal is obtained by the distortion of trigonal bipyramidal structure.
3Examples: CdCl 5 3- has trigonal bipyramidal structure. Ni(CN) 5 has got square
pyramidal structure.
3.5 Coordination number 6
This coordination number is the most important and commonest of all
coordination numbers for transition metal ions.
Complexes of the type, MA 6 has octahedral coordination and the symmetry is
O h . Other complexes of the type, MA5B, MA4B2, etc. are also known as
octahedral complexes even though they do not have O h symmetry.
M
4 Nomenclature of coordination compounds
4.1Symbol of complexes
When we want to write the complex using symbols, the symbol of the central
metal atom is placed first. Then symbols of the anionic ligands are written in
alphabetical order followed by the symbols of neutral ligands also in alphabetical
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order. Then the formula of the complex molecule or ion is enclosed within square
brackets [ ]. Example: [CoCl(NO 2 )(NH 3 ) 4 ]+
4.2 Names of complexes
While writing the names, at first, the ligands are listed alphabetically without
considering whether they are charged or neutral or how many of a particular
type of ligand is present. In other words, the prefixes such as di, tri, etc. should
not be considered for alphabetical order. Then the name of the central metal
atom is written.
4.3 Oxidation state
The oxidation state of the central metal atom is indicated in Roman numerals
within parentheses immediately after the metal name without leaving any space
between the two or charge on the entire ion can be indicated by Arabic numeral
within in parentheses without leaving any gap.
4.4 Isomers
The geometric isomers are denoted by prefixes in italics such as cis, trans,
fac, mer etc.
4.5 Special ending
If it is a complex anion, the name should end in –ate.
If the complex is a cation or neutral molecule, there is no special ending.
4.6 Prefixes for number of ligands
In simple cases, prefixes such as di, tri etc. are used to indicate two, three etc.
ligands respectively. In complicated cases or when the name of the ligand itself
involves such prefixes, bis, tris, tetrakis, pentakis etc. are used to indicate two,
three, four, five etc. ligands. The complex ligands are placed in parentheses
immediately after the prefix without leaving any space in between.
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4.7 Vowels in the name of ligands
Vowels should not be removed from the names or hyphen should not be
used. example: tetraammine is correct, while tetrammine is wrong.
4.8 Ligand name ending
Negative ligands should have - o ending. Example: chloro, cyano, sulphato etc.
Positive and neutral ligands have no special ending.
Examples:
Rule 1- symbols
[CoCl(NH3)5]Cl2
Correct
[Co(NH3)5Cl]Cl2
Wrong (order of ligands is incorrect)
Rule 2 – names
[CoCl(NH3)5]Cl2
pentaamminechlorocobalt(III) chloride;
correct
chloropentaamminecobalt(III) chloride; wrong (alphabetical order of ligands not
followed;(penta should not be considered)
Rule 3 – oxidation state
[CoCl(NH3)5]Cl2
pentaamminechlorocobalt(III) chloride;
correct
pentaamminechlorocobalt(2 +) chloride;
correct
pentaamminechlorocobalt (III) chloride; wrong (space between name and
parentheses is not allowed)
Rule 4 – Isomers
Cl
NH3
Cl
Pt
Cl
NH3
Pt
NH3
cis-Dichlorodiammineplatinum(II)
H3N
Cl
trans-Dichlorodiammineplatinum(II)
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Rule 5 – Special ending
If the complex is an anion, the name should end in -ate
Example: K4[Fe(CN)6]
Potassium hexacyanoferrate(II) or potassium hexacyanoferrate(4 -)
Rule 6 – prefixes for special ligands
Prefixes such as bis, tris etc. are used for special ligands
Example: Na3[Ag(S2O3)2]
Sodium
bis(thiosulfato)argentate(I)
or
Sodium
bis(thiosulfato)argentite(3-)
[Co(en)3]2(SO4)3
Tris(ethylenediamine)cobalt(III) sulfate or Tris(ethylenediamine)cobalt(3+) sulfate
Rule 7 – vowels in the names of ligands not to be deleted
[Co(NH3)6]Cl3
Hexaamminecobalt(III) chloride
correct
Hexaminecobalt(III) chloride
wrong
Rule 8 – Ligand name ending
Negative ligands should end in – o
Cl-
-
chloro
SO42- - sulfato
NO3- - nitrato
NO2- - nitrito
PO43-- phosphato
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5 References
1. “Inorganic Chemistry: Principles of Structure and Reactivity”, James
E.Huheey, Ellen A.Keiter, Richard L.Keiter, Okhil K.Medhi, Pearson
Education, Delhi, 2006
2. “Inorganic Chemistry”, Shriver and Atkins, 3/e, Oxford University Press,
2002,
3. “Concise Inorganic Chemistry”, 5/e, Blackwell Science, 2005,
4. “Concepts and Models of Inorganic Chemistry”, 3/e, John Wiley & Sons
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