Engineering Chemistry S Datta

304 ENGINEERING CHEMISTRY
d
z 2
d
x–y 2 2
e g
Energy Increasing
d
z 2
d d d xy yz zx
+ 0.6 o
dx–y
2 2
d d d
– 0.4 o
o or 10 D q
xy yz zx
t 2g
(a) (b) (c)
Fig. 13.10 Crystal Field Splitting in an octahedral complex. (a) The five degenerate d-orbitals in
the isolated central metal ion. (b) Hypothetical degenerate d-orbitals at a higher
energy level if all the ligands approach the metal ion at an equal distance from each
of the d-orbitals. (c) Crystal field splitting of d-orbitals into t 2g
and e g
sets by electrostatic
field of the six approaching ligands.
In the free metal ion all the five d-orbitals are degenerate (orbitals of same energy) as
shown in Fig. 13.10 (a). When the six ligands approach the central metal atom along the axes
they exert an electrostatic force of repulsion on the d-electrons of outermost shell i.e., the
d-electrons are repelled by lone pair of electrons of the ligands. This repulsion force raises the
degenerate d-orbitals of the metal to a higher energy state as shown in Fig. 13.10 (b) which is
considered as a hypothetical state. Since the lobes of d z
2 and d x
2 y
2 orbitals called as e
−
g
set of
orbitals fall directly at the path of the approaching ligands, the electrons in these orbitals
experience a greater force of repulsion exerted by the electrons of the ligands than that by the
electrons of the d xy
, d y2
and d zx
orbital called as t 2g
set of orbitals which are directed in space
along x, y and z axes. Hence, under the influence of the approaching ligands the orbitals d x
2 y
2
−
and d z
2 exist in the higher energy levels compared to d xy
, d yz
and d zx
. The separation of five d-
orbitals in the t 2g and e g
sets of different energy states is known as crystal field splitting.
The energy difference between e g
and t 2g
sets of orbitals is denoted by ∆ o
or 10D q
where ‘o’
indicates an octahedral arrangement of ligands around the central metal ion. ∆ o
is called crystal
field stabilisation energy (CFSE). It can be shown that when an octahedral complex is
formed the energy of the t 2g
set is decreased 0.4∆ o
while that of e g
set is increased by 0.6∆ o
as
shown in Fig. 13.10.
Strong and Weak Ligands
Strong ligands can split the five d-orbitals into t 2g
and e g
sets strongly i.e., there splitting
power is high whereas the weak ligands can split the set of orbitals weakly i.e., there splitting
power is weak. Strong ligands confer higher value of ∆ o
and weak ligands confer lower values
to ∆ o
. The following is the arrangement of ligands with the splitting power of increasing order.
This order is known as spectrochemical series.
C-donor > N-donor > O-donor > X-donor
Examples of C-donor = CN –
N-donor = NO – 2
, NH 3
O-donor = H 2
O, OH –
X-donor = F – > Cl – > Br – > I –
Distribution of d-electrons in t 2g
and e g
sets of orbitals—low spin and high spin
complexes.
• The strong ligands i.e., ∆ o
when high, force the electrons to form a pair in the lower t 2g
set of orbitals and thus the number of unpaired electrons is reduced, so the resultant