Coordination compounds result from the combination of two or more stable molecular compounds. They retain their identity in solid and solution states and have properties different from their constituents. There is a central metal atom that provides vacant orbitals and ligands that donate electron pairs. Coordination compounds differ from double salts in that double salts exhibit characteristics of all constituent ions but coordination compounds do not. Terms related to coordination compounds include complex ion or entity, central atom or ion, and ligands. Ligands are electron donating species bound to the central metal and can be unidentate, bidentate, or polydentate.

1. Coordination
Compounds
Basics to Advance

2. Introduction
A molecular compound resulting from the combination of two or more
stable molecular compounds and retains its identity in the solid state as
well as in solution and its properties are totally different from the
constituents present.
There is a central metal which provide vacant orbitals and ligands which
donates electron pairs.
[Pt(NH3)4]2+ [Fe(CN)6]4-

3. Co-ordination compounds v/s double salts
• Double salts are prepared in the same way as coordination
compounds are prepared but they give characteristics of all the ions
present but coordination compounds do not do so .
• Example of double salt – Mohr’s salt FeSO4(NH4)2SO4.6H2O
Potash alum - KAl(SO4)2.12H2O

4. Terms Related to Coordination Compounds
1. Complex ion or Coordination Entity
It is an electrically charged species in which central metal atom or ion is
surrounded by number of ions or neutral molecules.
(i) Cationic complex entity- It is the complex ion which carries positive
charge. e.g., [Pt(NH3)4]2+
(ii) Anionic complex entity - It is the complex ion which carries negative
charge. e.g., [Fe(CN)6]4-

5. Central Atom or Ion
1. The atom or ion to which a fixed number of ions or groups are bound is
central atom or ion.
2. It is also referred as Lewis acid. e.g., in (NiCI2(H2O)4]. Ni is central
metal atom.
3. It is generally transition element or inner-transition element.

6. Ligands
• Ligands is electron donating species (ions or molecules) bound to the
Central atom in the coordination entity.
These may be charged or neutral.
LIgands are of the following types:
1. Unidentate: It is a ligand, which has one donor site, i.e., the ligand
bound to a metal ion through a single donor site.
NEUTRAL - H2O aqua
NH3 ammine
CO carbonyl
NO nitrosyl
CS thiocarbonyl

7. Bidentate ligands
• F–Fluorido , I – iodo, Cl–Chlorido,
• OH–hydroxo, O2
2-peroxo, O2
–superoxo,
• CH3COO–acetato, CO3
2-carbonato,
• SO4
2-sulphato, SO3
2-sulphito,
• SCN–thiosulphato,-S,
• NCS–isothiocyanato-N

8. Bidentate ligands
These ligands have two donor atoms which can attach to a single
metal cation or atom.
A bidentate ligand forms one 5- or 6- membered ring with a metal ion.
Some examples of bidentate ligands are- oxalate ion , Ethane-1,2-
diamine, Sulphate ion, Carbonate ion etc.

9. Polydentate ligand
Teridentate ligand - Diethylenetriamine (diene)
Tetradentate ligand- Triethylenetetraamine (triene)
Hexadentate ligand - Ethylenediaminetetraacetateion (EDTA)

10. Chelation and denticity

11. Co-ordination number and oxidation no.
1. [PtCl6]2
2. [Co(NH3)4Cl2]+
3. [Cr(NH3)3(H2O)3]Cl3
4. K4[Fe(CN)6]
5. [Ni(CO)4]

12. Rules for nomenclature
 Name of the compound is written in two parts (i) name of cation, and
(ii) name of anion.
 The cation is named first in both positively and negatively charged
coordination complexes.
 The dissimilar ligands are named in alphabetical order before the
name of central metal atom or ion.
 For more then one similar ligands. the prefixes di, tri, tetra, etc are
added before its name.
 If the di, tri, etc already appear in the complex then bis, tris, tetrakis are
used.

13. • If the complex part is anion, the name of the central metal ends with
suffix ‘ate’.
• Names of the anionic ligands end in ‘0’, names of positive ligands end
with ‘ium’ and names of neutral ligands remains as such. But exception
are there as we use aqua for H2O, ammine for NH3, carbonyl for CO
and nitrosyl for NO.
• Oxidation state for the metal in cation, anion or neutral coordination
compounds is indicated by Roman numeral in parentheses.
• The name of the complex part is written as one word.
• If the complex ion is a cation, the metal is named same as the element.
• The neutral complex molecule is named similar to that of the complex
cation.

14. Nomenclature
1. [Co(H2CH2CH2H2)3]2(SO4)3
2. [Ag(NH3)2] [Ag(CN)2]
3. K4 [Fe(CN)6]
4. [Co(NH3)5(ONO)]Cl

16. Structural isomerism
1.Structural Isomerism
In this isomerism. isomers have different bonding pattern.
Different type of structural isomers are-
(i) Linkage isomerism - This type of isomerism is shown by the
coordination compounds having ambidentate ligands. e.g.,
[Co(NH3)5(NO2)]Cl and [Co(NH3)5(ONO)]Cl

17. (ii) Coordination isomerism -This type of isomerism arises from the
interchange of ligands between cationic and anionic complexes of
different metal ions present in a complex, e.g.,
[Cr(NH3)6) [CO(CN)6]and [CO(NH3)6] [Cr(CN)6]
(iii) Ionisation isomerism- This isomerism arise due to exchange of
ionisable anion with anionic ligand.

18. (iv) Solvate isomerism- This is also known as hydrate isomerism.
In this isomerism, water is taken as solvent. It has different number of
water molecules in the coordination sphere and outside it.
e.g. [Co(H2O)6]CI3, [Co(H2O)4C12]Cl·2H2O, [Co(H2O)3Cl3]. 3H2O

19. 2. Stereoisomerism
Stereoisomers have the same chemical formula and chemical bonds
but they have different spatial arrangement. These are of two types :
(i) Geometrical isomers are of two types i.e., cis and trans isomers.
This isomerism is common in complexes with coordination number 4
and 6.

20. Geometrical isomerism in complexes with coordination number 4
(i) Tetrahedral complexes do not show geometrical isomerism.
(ii) Square planar complexes of formula [MX2L2] (X and L are unidentate)
show geometrical isomerism.
The two X ligands may be arranged adjacent to each other in a cis
isomer, or opposite to each other in a trans isomer,

21. Geometrical isomerism in complexes with coordination number 6
Octahedral complexes of formula [MX2L4], in which the two X ligands may
be oriented cis or trans to each other, e.g., [Co(NH3)4Cl2)+.

22. (ii) Optical isomerism -These are the complexes which have chiral
structures.
It arises when mirror images cannot be superimposed on one another.
These mirror images are called enantiomers.
The two forms are called dextro (d) and laevo (l) forms.
Tetrahedral complexes with formula [M(AB)2] show optical isomers
and octahedral complexes (cis form) exhibit optical isomerism

23. Werner’s Theory

24. Valence Bond Theory (VBT)
1. The central metal cation or atom makes available a number of vacant
s, p and or d-orbitals equal to its coordination number to form
coordinate covalent bonds with ligands.
2. These vacant atomic orbitals of metal are hybridised to form a new set
of equivalent bonding orbitals, called hybrid orbitals. These orbitals
have the same geometry, the same energy and definite directional
properties.
3. The bonding in metal complexes arises when a filled ligand orbital
containing a lone pair of electrons overlaps a vacant hybrid orbital on
the metal cation or atom to form a coordinate covalent bond.

25. 4. Each ligand has at least one orbital containing a lone pair of electrons.
Pauling classified the ligands into two categories (i) Strong ligands like
CN–, CO– etc. (ii) weak ligands like F–, Cl– etc.
5. Strong ligands have a tendency to pair up the d-electrons of a metal
cation or atom to provide the necessary orbitals for hybridization. On
the other hand, weak ligands do not have a tendency to pair up the d-
electrons.
6. The d orbital used in hybridization may be either inner (n-1) d-
orbitals or outer n d-orbitals. The complex formed by inner (n-1) d-
orbitals, is called inner orbital complex whereas the complex formed
by outer d-orbital is called outer orbital complex.
7. If unpaired electrons are present within the complex, then complex is
paramagnetic in nature while if all the electrons are paired then
complex is diamagnetic in nature.

26. Inner orbital complex
Examples of Octahedral complexes
(a) Inner Orbital Complexes:
[Co(CN)6]3- ion:

27. Outer orbital complex
(b) Outer Orbital Complexes:
[Fe(F)6]3- ion:

28. Examples of tetrahedral complexes
[NiCl4]2- ion:

29. Crystal field theory

30. Thank you