This presentation gives information about coordination compounds and its theory and nomenclature

1. THEORY OF COORDINATION
COMPOUNDS
By Mohammed Ismail A

2. COORDINATION COMPOUND
A coordination complex is the product of a Lewis
acid-base reaction in which neutral molecules or
anions (called ligands) bond to a central metal atom
(or ion) by coordinate covalent bonds.

3. COORDINATION BOND
 A coordinate bond(dative covalent bond) is a
covalent bond in which both elctrons come from
same atom
 A coordinate bond is shown by an arrow.the arrow
points from the atom donating the lone pair to the
accepting atom

4. TERMINOLOGIES
1. Lewis Acid
1. All electron acceptors are lewis acids.
2. Lewis Base
1. All electron donors are lewis base.
3. Central metal ion
1. In the complex ion an acceptor accepts a pair of
electrons from the donor atoms. The acceptor is
usually a metal / metal ion to which one (or) more of
neutral molecules (or) anions are attached. The
acceptor metal cation is referred to as central metal
cation. Hence, central metal cation in a complex
serves as a lewis acid.

5. TERMINOLOGIES CONTD.
 Oxidation state
 This number denotes the charge, explaining the number of
electrons it has lost to form the cation. It is oxidation number
that denotes the charge, if the central metal atom would have
if all the ligand in the complex were removed along with their
electron pairs that were shared with the central atom. It is
usually represented by Roman Numeral.
 Ligand (Latin word meaning to bind)
 A ligand is an ion (or) a molecule capable of functioning as an
electron donor. Therefore the neutral molecules or ions which
are directly attached to the central metal ion are called as
ligand (or) coordination groups. These coordination groups or
ligands can donate a pair of electrons to the central metal ion
(or) atom. Hence, in a complex compound ligands act as
Lewisbases.

6. TERMINOLOGIES CONTD.
 Coordination sphere
 In a complex compound, it usually, central metal ion and the
ligands are enclosed with in square bracket is called as
coordination sphere. This represents a single constituent unit.
The ionisable species are placed outside the square bracket.
 These ions do not ionise to give the test for constituent ions.
 Coordination number
 The coordination number of a metal ion in a complex can be
defined as the number of ligand donor atoms to which the
metal is directly bonded. Numerically coordination number
represents the total number of the chemical bonds formed
between the central metal ion and the donor atoms of the
ligands. For example in K4[Fe(CN)6] the coordination number
of Fe(II) is 6 and in [Cu(NH3)4]SO4 the coordination number
of Cu(II) is 4.

7. TYPES OF LIGAND
 When a ligand is bound to a metal ion through a single
donor atom, as with - Cl , H2O or NH3, the ligand is said
to be unidentate. Whenever a single coordinating group
(or) ligand occupies two (or) more coordination position
on
 the same central metal ions, a complex possessing a
closed ring is formed. Such ligands are called
polydentate ligands. When a single ligand has two
coordinating positions,it is called bidentate ligand and
when there are three coordinating positions available, it
is called a tridentate ligand and so on. For example,
ethylenediamine is a bidentate ligand because it has two
amino groups each of which can donate a pair of
electrons.

8. NAME OF LIGAND
 Positive ligands
 The positive ligands are named with an ending -ium.
 Neutral ligands
 The neutral ligands are named as such without any special
name. But water is written as 'aqua : Ammonia is written as
ammine. Note that two m's to distinguish from organic
amine CO-Carbonyl, NO-Nitrosyl, NH2 - CH2 - CH2 - NH2-
ethylenediamine (en), Pyridine C5H5N.
 Negative Ligands
 Negative ligands end in suffix 'O'.
Example
F--Fluoro, Cl--Chloro, C2O42--Oxalato, CN--Cyano, NO2--
Nitro, Br--Bromo, SO42--Sulphato, CH3COO--acetato
CNS--thiocyanato, NCS--isothiocyanato, S2O32--
thiosulphato.

9. CONTINUED
 Chelates
 If a ligand is capable of forming more than one bond
with the central metal atom (or) ion then the ring
structures are produced which are known as metal
chelates. Hence the ring forming group are described as
chelating agents (or) polydentate ligands.

10. THEORY OF COORDINATION COMPLEX
Werner’s theory :
Alfred Werner studied the structure of coordination
complexes and put forward his ideas in the year 1893
which were known as 'Werner's coordination theory.
Experimental verification:
when silver nitrate was added to CoCl3·6NH3, all
the three chloride ions were converted to silver chloride.
However, when silver nitrate was added to CoCl3·5NH3,
only two mole of silver chloride was formed. When
CoCl3·4NH3 was treated with silver nitrate, one mole of
silver chloride was formed. Based on his observations,
Werner postulated the following theory

11. POSTULATES OF WERNER’S THEORY
1.Every metal atom has two types of valencies
i) Primary valency or ionisable valency
ii) Secondary valency or non ionisable valency
2.The primary valency corresponds to the oxidation state of the metal ion.
The primary valency of the metal ion is always satisfied by negative ions.
3.Secondary valency corresponds to the coordination number of the metal
ion or atom. The secondary valencies may be satisfied by either negative ions or
neutral molecules.
4.The molecules or ion that satisfy secondary valencies are called ligands.
5.The ligands which satisfy secondary valencies must project in definite
directions in space. So the secondary valencies are directional in nature whereas
the primary valencies are non-directional in nature.
6.The ligands have unshared pair of electrons. These unshared pair of electrons
are donated to central metal ion or atom in a compound. Such compounds are
called coordination compounds.

12. WERNER’S REPRESENTATION
Werner represented the first member of the series
[Co(NH3)6]Cl3 as follows.
In this representation, the primary valency
(dotted lines) are satisfied by the three chloride ions.
The six secondary valencies (solid lines) are satisfied
by the six ammonia molecules.

13. LIMITATIONS OF WERNER’S THEORY
 It failed to explain why all elements don’t form
coordination compounds.
 It failed to explain the directional properties of
bonds in coordination compounds.
 It does not explain the colour, and the magnetic and
optical properties shown by coordination
compounds.

14. VALENCE BOND THEORY
Valence bond theory, primarily the work of Linus Pauling
regarded bonding as characterized by the overlap of atomic or
hybrid orbitals of individual atoms.
The postulates of valence bond theory
 The central metal atom/ion makes available a number of
vacant orbitals equal to its coordination number.
 These vacant orbitals form covalent bonds with the ligand
orbitals.
 A covalent bond is formed by the overlap of a vacant metal
orbital and filled ligand orbitals. This complete overlap leads to
the formation of a metal ligand,  (sigma) bond.
 A strong covalent bond is formed only when the orbitals
overlap to the maximum extent. This maximum overlapping is
possible only when the metal vacant orbitals undergo a
process called 'hybridisation'. A hybridised orbital has a better
directional characteristics than an unhybridised one.

15. The following table gives the coordination number,
orbital hybridisation and spatial geometry of the more
important geometrics
Coordination
number
Type of
hybridisation
Geometry
2 sp Linear
4 sp3 Tetrahedral
4 dsp2 Square planar
6 sp3d2 Octahedral
6 d2sp3 Octahedral

16. MAGNETIC MOMENT

17. APPLICATIONS OF VALENCE BOND THEORY

18. CONTINUED.
Octahedral complex

19. DEFECTS OF VALENCE BOND THEORY
Although VB theory was the principal way in which
chemist visualized coordination compounds until the
1950s, it has fallen into disfavour due to its inability to
account for various magnetic, electronic and
spectroscopic properties of these compounds.

20. NOMENCLATURE OF COORDINATION
COMPOUNDS
The coordination compounds are named in the
following way.
 To name a coordination compound, no matter
whether the complex ion is the cation or the
anion, always name the cation before the anion.
(This is just like naming an ionic compound.)
 In naming the complex ion:
1. Name the ligands first, in alphabetical order, then
the metal atom or ion. Note: The metal atom or ion is
written before the ligands in the chemical formula

21. CONTINUED
2. The names of some common ligands are listed in
Table 1.
 For anionic ligands end in "-o"; for anions that end
in "-ide"(e.g. chloride), "-ate" (e.g. sulfate, nitrate),
and "-ite" (e.g. nirite), change the endings as
follows: -ide -o; -ate -ato; -ite -ito
 For neutral ligands, the common name of the
molecule is used e.g.
H2NCH2CH2NH2 (ethylenediamine).
 Important exceptions: water is called ‘aqua’,
ammonia is called ‘ammine’, carbon monoxide is
called ‘carbonyl’, and the N2 and O2 are called
‘dinitrogen’ and ‘dioxygen’.

22. TABLES
Anionic
Ligands
Names Neutral
Ligands
Names
Br- bromo NH3 ammine
F- fluoro H2O aqua
O2- oxo NO Nitrosyl
OH- Hydroxo CO Carbonyl
CN- cyano O2 dioxygen
C2O4
2- oxalato N2 dinitrogen
CO3
2- carbonato C5H5N pyridine
CH3COO- acetato H2NCH2C
H2NH2
ethylenediamine
Table 1. Names of Some Common Ligands

23. CONTINUED
3. Greek prefixes are used to designate the number
of each type of ligand in the complex ion. If the ligand
already contains a Greek prefix (e.g. ethylenediamine) or
if it is polydentate ligands (ie. can attach at more than one
binding site) the prefixes bis-, tris-, tetrakis-, pentakis-, are
used instead. The numerical prefixes are listed in Table 2.
4. After naming the ligands, name the central metal. If
the complex ion is a cation, the metal is named same as
the element. For example, Co in a complex cation is call
cobalt and Pt is called platinum. If the complex ion is an
anion, the name of the metal ends with the suffix –ate. For
example, Co in a complex anion is called cobaltate and Pt
is called platinate. For some metals, the Latin names are
used in the complex anions e.g. Fe is called ferrate (not
ironate).

24. Numbe
r
Prefix Num
ber
Prefix Num
ber
Prefix
1 mono 5 penta
(pentakis)
9 nona
(ennea)
2 di (bis) 6 hexa
(hexakis)
10 deca
3 tri (tris) 7 hepta 11 undeca
4 tetra
(tetrakis
)
8 octa 12 dodeca
Table 2. Numerical Prefixes

25. Name of Metal Name in an Anionic
Complex
Iron Ferrate
Copper Cuprate
Lead Plumbate
Silver Argenate
Gold Aurate
Tin Stannate
Table 3: Name of Metals in Anionic Complexes

26. CONTINUED
5. Following the name of the metal, the oxidation
state of the metal in the complex is given as a Roman
numeral in parentheses.
C. To name a neutral complex molecule, follow
the rules of naming a complex cation
Remember: Name the (possibly complex)
catiom BEFORE the (possibly complex) anion.
 For historic reasons, some coordination compounds
are called by their common names. For example,
Fe(CN)6
3- and Fe(CN)6
4- are named ferricyanide
and ferrocyanide respectively, and Fe(CO)5 is
called iron carbonyl.

27. EXAMPLES
1. [Cr(NH3)3(H2O)3]Cl3
Answer: triamminetriaquachromium(III) chloride
2. [Pt(NH3)5Cl]Br3
Answer: pentaamminechloroplatinum(IV) bromide
3. [Pt(H2NCH2CH2NH2)2Cl2]Cl2
Answer: dichlorobis(ethylenediamine)platinum(IV)
chloride
4. [Co(H2NCH2CH2NH2)3]2(SO4)3
Answer: tris(ethylenediamine)cobalt(III) sulfate
5. K4[Fe(CN)6]
Answer: potassium hexacyanoferrate(II)

28. CONTINED.
6. Na2[NiCl4]
Answer: sodium tetrachloronickelate(II)
7. Pt(NH3)2Cl4
Answer: diamminetetrachloroplatinum(IV)
8. Fe(CO)5
Answer: pentacarbonyliron(0)
9. (NH4)2[Ni(C2O4)2(H2O)2]
Answer: ammonium
diaquabis(oxalato)nickelate(II)
10.[Ag(NH3)2][Ag(CN)2]
Answer: diamminesilver(I) dicyanoargentate(I)

29. CAN YOU GIVE THE MOLECULAR FORMULAS OF
THE FOLLOWING COORDINATION COMPOUNDS?
 1. hexaammineiron(III) nitrate
 2. ammonium tetrachlorocuprate(II)
 3. sodium monochloropentacyanoferrate(III)
 4. potassium hexafluorocobaltate(III)

30. ANSWERS
 1. [Fe(NH3)6](NO3)3
 2. (NH4)2[CuCl4]
 3. Na3[FeCl1(CN)5]
 4. K3[CoF6]