Most
of the transition metal compounds are coloured both in the solid state and
in aqueous solution. This is because of the presence of incompletely
filled d-orbitals. When a transition metal compound is formed the degenerate d-orbitals of the metal split into
two sets, one having three orbitals dxy, dyz and dxz called t2g orbitals with lower energy and the other having two
orbitals dx2 –y2 and dz2 called eg orbitals with slightly higher energy in an octahedral field.
This is called crystal field splitting When white light falls on these
compounds, some wavelength is absorbed for promotion of electrons from one
set of lower energy orbitals to another set of
slightly
higher energy within the same d-subshell. This is
called d-d transition. The remainder light is reflected which has a particular
colour.
The colours of some 3d metal
ions:
SN
|
d-configuration
|
Examples with colour
|
1
|
d0 (No d-d transition)
|
|
2
|
d1
|
Ti3+
(3d1) Purple, V+4(3d1)
Blue
|
3
|
d2
|
V+3(3d1)
Green
|
4
|
d3
|
Cr3+
(3d3) Violet green
|
5
|
d4
|
Mn+3(3d4)
Violet , Cr2+ (3d4) Blue
|
6
|
d5
|
Mn+2(3d5)
Pink, Fe+3(3d5) Yellow
|
7
|
d6
|
Fe+2(3d6)
Brown , Co+2(3d6) Green ,
|
8
|
d7
|
|
9
|
d8
|
Ni+2(3d8)
Green
|
10
|
d9
|
Cu+2(3d9)
Blue
|
10
|
d10
|
Sc+3(3d0)
colourless
|
ILLUSTRATION (1): The mechanism
of light absorption in coordination compounds is that photons of appropriate
energy can excite the coordination entity from its ground state to an excited
state. Consider [Ti(H2O)6]3+
In which Ti(+3) ion has one electron in d sub shell ( in
lower energy t2g
d- orbital) . In aqueous solution, [Ti(H2O)6]3+. Appear as purple due to
the absorption of light from visible range ( green and yellow portion)
resulting d-d transition ( electron jump
from t2g level to eg level) as result complex has complementary ie purple .
The
variety of color among transition metal complexes has long fascinated the
chemists.
ILLUSTRATION (2) : Aqueous solutions of [Fe(H2O)6]3+
are red, [Co(H2O)6]2+ are pink, [Ni(H2O)6]2+ are green, [Cu(H2O)6]2+ are blue and [Zn(H2O)6]2+
are colorless. Although the octahedral [Co(H2O)6]2+
are pink, those of tetrahedral [CoCl4]2- are blue. The green color of [Ni(H2O)6]2+
turns blue when ammonia is added to give [Ni(NH3)6]2+. Many of these facts
can be rationalized from CFT.
Related Question:
(1) Although both [Mn(H2O)6]2+ and [FeF6]3- have a d5 configuration and high-spin complexes. But the dilute solutions of Mn2+ and Fe +3 complexes are therefore colorless. Why?
Related Question:
(1) Although both [Mn(H2O)6]2+ and [FeF6]3- have a d5 configuration and high-spin complexes. But the dilute solutions of Mn2+ and Fe +3 complexes are therefore colorless. Why?
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