Coordination compounds

Addition or Molecular Compounds definition

When solutions of two or more salts in simple molecular ratio are mixed together & allowed to crystallized together, crystals of new compound are obtained. This new compound is called addition or molecular compound.

Types of addition/molecular compound

two types
double salts, coordination compounds

Double salts

These compounds are stable in solid state only but are broken into their individual compounds when dissolved in water or any other ionic solvent. Their solutions have the same properties as a mixture of the individual compounds.

Example of double salt

Mohr's salt: FeSO₄.(NH₄)₂SO₄.6H₂O
Potash alum: K₂SO₄. Al₂(SO₄)₃. 24H₂O

physical and chemical properties of double salt

The physical and chemical properties of a double salt are the same as those of their individual compounds.

Coordination compounds

▪These compounds retain their identities in the solid sate as well as when dissolved in water or any other ionic solvent.
▪Their properties are completely different from those of their individual constituents.

Example of coordination compound

Hexaammine nickel chloride, [Ni(NH3)6]Cl2

Complex Ion definition

A complex ion can be defined as an electrically charged radical which is obtained by the combination of a metal cation with one or more neutral molecules or simple anions (known as ligands).

complex ion example

▪[Ni(NH3)6]+2

▪[Fe(CN)6]4-

What is a Coordination Compound?

▪A coordination complex is the product of a Lewis acid-base reaction in which neutral molecules or anions (called ligands) are bonded to a central metal atom (or ion) by coordinate covalent bonds.
▪Coordinate covalent bonds are obtained when a Lewis base (an electron donor or giver) donates a pair of electrons to a Lewis acid (an electron acceptor) to give a so-called adduct. The process of forming a dative bond is called coordination. The electron donor acquires a positive formal charge, while the electron acceptor acquires a negative formal charge.

ligands or coordinating groups

The neutral molecules or ions (usually anions) which are attached with the central ion in complex compounds are called ligands or coordinating groups

example of ligand

in the complex ion [Fe(CN)6]3- the six CN- ions which are attached with the central Fe3+ ion act as ligands. In most of the complex compounds, the ligands act as Lewis bases (electron pair donors) and the central metal ion acts as a Lewis acid (electron pair acceptor). In most of the complex compounds the ligands donate one or more electron pairs to the central metal ion.

Donor atom definition

In a ligand the atom which actually donates the electron pair to the central metal ion is called donor atom

How are the ligands attached?

The ligands are attached with the central metal ion through their donor atom (or atoms).

geometric shapes of ligand

linear, tetrahedral, square planar, and octahedral

linear shape of ligand coordinated number and example

2
CuCl2, [Ag(NH3)2]+ , [AuCl2]

Square planar shape of ligand coordinated number and example

4
[Ni(CN)4]2- , [PdCl4]2- , [Pt(NH3)4]2+ , [Cu(NH3)4]2+

tetrahedral shape of ligand coordinated number and example

4
[Cu(CN)4]3- , [Zn(NH3)4]2+ , [CdCl4]2- [MnCl4]2-

octahedral shape of ligand coordinated number and example

[Cu(H2O)6]3+ , [V(CN6)]4- , [Cu(NH3)4Cl2]+ , [Co(en)3]3+

What is a ligand

A ligand is an ion or molecule that binds to a central metal atom to form a complex.

Types of Ligands

1.Monodentate or unidentate ligands
2.Polydentate ligands

Monodentate or unidentate ligands definition and example

The ligands which have only one donor atom or are coordinated through one electron pair are called monodentate or unidentate ligands
Example: Ammonia, water

Polydentate ligands definition and example

The ligands having two or more donor atoms are called polydentate ligands.
Example: Ethane-1,2-diamine, 1,4,7-triazaheptane (also known as diethylenetriamine)

Coordination number definition

The number of ligands which are directly attached with the central metal atom/ion is known as the coordination number of that metal atom/ion

Coordination sphere definition

The ligands which are directly linked with the central metal ion are enclosed in square bracket which is called coordination sphere

What does coordination sphere consist of?

coordination sphere consists of the central metal atom/ion and the ligands attached with it
Thus in the complex compound, Hexamminecobalt chloride, [Co(NH3)6]Cl3, the coordination sphere consists of one Co3+ ion and six NH3 molecules which are acting as ligands.

Chelated complex definition

When all the donor atoms of a polydendate ligand get coordinated with the same metal ion, then a complex compound is formed which contains one or more rings in its structure. Hence it is called Chelated complex or Chelate.

Chelation definition and example

The process of the formation of a chelated complex is called Chelation
Example: Ethylenediamine ligand chelating to a metal with two bonds

How are isomers defined as?

Isomers are defined as compounds having the same molecular formula but different structures.

types of isomerism

1. Ionization isomerism
2. Hydrate isomerism
3. Linkage isomerism
4. Coordination isomerism
5. Ligand isomerism
6. Geometrical isomerism
7. Optical isomerism

Explain ionization isomerism

Complexes containing the same chemical composition but giving different ions in solution are called ionization isomers. Thus [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br are ionization isomers. In solution these compounds give tests of ‎SO42- and Br- ions respectively. The amine groups within the coordination sphere are not ionizable.

Explain hydrate isomerism

Isomerism exhibited by complex compounds due to the different mode of combination of water molecules is called hydrate isomerism. Thus [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2.H2O are hydrate isomers. These hydrate isomers show difference in physical and chemical properties and also in color

Explain linkage isomerism

Linkage isomerism or structural isomerism occurs when a coordinated group has two or more atoms which can function as electron donor

Explain geometric isomerism

This is also known as stereoisomerism or cis-trans isomerism. This is the most important type of isomerism in the co-ordination compounds and is a direct outcome of the arrangement of the coordinated groups in space. The geometrical isomerism is based on the geometry of the coordination complexes and is exhibited by compounds of 6-coordination numbers and also by other coordination numbers.

Explain optical isomerism

Optical isomers are mirror images of each other. They have similar physical and chemical properties but differ in rotating the plane of plane polarized light. Isomer which rotates the plane polarized light to the right is called dextro rotatory (d-form) and the isomer which rotates the plane polarized light to the left is called levo rotatory (l-form). Complex with a coordination number 4 and 6 show this type of isomerism.

Application of Chelate Formation

1. Application of chelates in analytical chemistry (Color Tests and Complexometric titrations & masking agents)
2. Formation of chelates in softening of water
3. Formation of chelates in medicines (removal of poisonous metals from the body)
4. Formation of chelates in living systems

Werner's Coordination Theory

(1) Every metal atom or ion exhibits two types of valencies: primary (ionizable) valency and secondary (non ionizable) valency.
(2) Every metal has a fixed number of secondary valencies.
(3) The primary valency of the central metal ion is satisfied by negative ions. Its attachment to the metal ion is shown by dotted lines. It corresponds to the oxidation state of the metal atom or ion and is ionizable. The secondary valency is satisfied by either negative ions or neutral molecules. Its attachment to the metal is shown by thick lines. It corresponds to the coordination number of the metal atom or ion and is non-ionizable. Every metal atom or ion wants to satisfy both its primary and secondary valencies. In order to meet this requirement, a negative ion may perform a dual function of satisfying both types of valences.
(4) The ligands which satisfy the secondary valencies are always directed towards fixed positions in space.

What did Sidgwick suggest?

Sidgwick suggested that after the ligands have donated certain number of electrons to the central metal ion through the L - M (Ligand-Metal) bonding, the total numbers of electrons in the central atom including those gained from the ligands during bonding is called effective atomic number (EAN) of the metal ion.

What does EAN represent?

Effective atomic number (EAN), number that represents the total number of electrons surrounding the nucleus of a metal atom in a metal complex.

Formula of EAN

(Z - x) + n X y

Factors affecting the stability of chelates

▪Group in the periodic table: The most stable complexes are formed by the cations of transition metals.
▪High positive charge
▪small cationic radius
▪unoccupied d orbitals