The crystal field theory attributes the colour of cooedination compounds to

Options

A. d-d transition
B. charge transfer spectrum
C. polarisation
D. All of these

Answer

A

Explanation

One of the most distinctive properties of transitio metal complexes is their wide range of colours. Colours of transtition metal ions in aqueous solution correspond to the presence of their aqua complexes. The color of a substance is due to the absorption of light of a specific wavelength in the visible part of the electromagnetic spectrum and transmission or reflection of the rest of the wavelengths. An object that absorbs all visible light appars balck. This implies that some of the visible spectrum is being removed form light as it passess through the sample of transition metal complex, so teh light that emerges is no longer white. The colour of the complex is complementary to that which is absorbed. Teh complementary colour is the colour generated form the wavelength left over, if green light is absorbed by the complex, it appears red. The color in the coordination compounds can be readily explained in terms of teh cryptal field theory. Consider the complex ion, `[Ti(H_(2)O)_(6)]^(3+)` which is violet in colour. This is an octahedral complex ion where the single 3d electron (`Ti^(3+)` is a `3d^(1)` system) in the metal is in the `t_(2g)` level in teh ground state of the complex. The next higher state available for the electron is the empty `e_(g)` level. When light is absorbed by the complex it excites the electron form `t_(2g)` level to the `e_(g)` level:
`t_(2g)^(1)e_(g)^(0) rarr t_(2g)^(0)e_(g_^(1)`
Consequently, the complex appears violet in colour. Since this excitation involves electron,s jump form one degenerate set of d orbitals to another set of d orbitals, it is commonly called d-d transition. It is important to note that in the absence of ligands, crystal field splitting to note that in the absence of ligands, crystal field splitting does not occur and hence the substance is colourless as there is no d-d transition. For example, removal of `H_(2)O` Ligands form `[Ti(H_(2)O)_(6)]C1_(3)` on heating renders it colourless. Similarly anhydrous `CuSO_(4)` is with, but `CuSO_(4).5H_(2)O` iws blue in colour.