Engineering Chemistry S Datta

300 ENGINEERING CHEMISTRY
Optical or Mirror Image Isomerism
Optical isomerism of 6- and 4-co-ordination complex compounds is shown in Fig. 13.6
and 13.7.
The complex [Cr(en) 2
Cl 2
] with co-ordination number 6 exhibits optical isomerism.
Mirror plane
en
Cl
Cl
en
Cr
Cr
en
Cl
Cl
en
(a)
(b)
Fig. 13.6
HC 3
As
HC 5 2
–
C6H4COO
S
(–)
OOC
HC 4 6
S
As
CH 3
C 2 H 5
(a)
Mirror plane
(b)
Fig. 13.7 Optically active arsenic compound with co-ordination number 4.
Theories of Bonding in Co-ordination Compounds
There are four distinct approaches to the theoretical treatment of bonding in co-ordination
compounds. These are: (i) Valence bond theory, (ii) Crystal field theory, (iii) Molecular orbital
theory and (iv) Ligand field theory.
In this context, the first two theories have been discussed.
Valence Bond Theory
This theory was developed by Pauling and deals with hybridisation of the complex. It
depends on the following assumptions:
• The central metal atom provides a number of empty orbitals that are equal to its coordination
number for forming the covalent bonds with ligand orbitals.
• The empty orbitals of the metal ion hybridise to give an equal number of equivalent
hybrid orbitals.
• Ligands are groups which can donate electron pairs to metal ions which appear as
electron pair acceptors.
..
Ag + + 2N H3 ⎯⎯→ [Ag(NH 3
) 2
] +
• An orbital of the ligand containing an unshared electron pair overlaps the hybridised
orbitals of the metal ion and this is regarded either as partial donation of the ligand
electrons to the metal ion or a sharing of that pair of electrons between the metal ion
and the ligand. This overlapping results in a σ-bond or possibly a π-bond.