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What is d-d transition in complexes and explain colour of complex by d-d transition ?

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.

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