Theories Of Coordination Compounds: Werner's theory, Valence bond theory

THEORIES OF COORDINATION COMPOUNDS

1 Werner's Theory

Alfred Werner (1866-1919) French born Swiss chemist founded the modern theory on coordination compounds. His theory and pioneering experimental work on metal complexes won for him the Nobel Prize for chemistry in 1913. Werner was the first inorganic chemist to be awarded the nobel prize in chemistry. He is considered at "Father of coordination chemistry".

Brief concepts of Werner's theory of coordination compounds

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.

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.

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.

Defects of Werner's theory

Werner's theory describes the structures of many coordination compounds successfully. However, it does not explain the magnetic and spectral properties.

2 Valence bond theory (VB 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

1.           The central metal atom/ion makes available a number of vacant orbitals equal to its coordination number.

2.     These vacant orbitals form covalent bonds with the ligand orbitals.

3.     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, s (sigma) bond.

4.           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.

The following table gives the coordination number, orbital hybridisation and spatial geometry of the more important geometrics.

Coordination number - Types of hybridization - Geometry

    2  -    sp   -    linear

    2  -    sp3  -    tetrahedral

    2  -    dsp2  -   square planar

    2  -    d2sp3  -    octahedral

    2  -    sp 3 d 2 -    octahedral

Magnetic moment

A species having atleast one unpaired electron, is said to be paramagnetic.

It is attracted by an external field. The paramagnetic moment is given by the following spin-only formula.

ìs = rt(n(n+ 2) )BM

ms = spin-only magnetic moment

n = number of unpaired electrons

BM = Bohrmagneton,theunitwhichexpressesthemagneticmoment.When the species does not contain any unpaired electron, it is diamagnetic.

Number of unpaired electrons                   Spin-only moment (BM)

1                                             root(1(1+ 2)) = 1.73

2                                             root(2(2 + 2)) =2.83

3                                             rt(3(3+ 2))= 3.87

4                                             rt(4(4+2)) = 4.90

5                                             rt(5(5+ 2))=5.92

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.