[harami]

I dont get the difference between the two, dont they both deal with g-u transitions being allowed and those such as d-d (g-g) not being alllowed?

[mdlhvn]

[edit] Charge transfer complexes and color
Many metal complexes are colored due to d-d electronic transitions. Visible light of the correct wavelength is absorbed, promoting a lower d-electron to a higher excited state. This absorption of light causes color. These colors are usually quite faint, though. This is because of two selection rules:

The spin rule: ? S = 0
On promotion, the electron should not experience a change in spin. Electronic transitions which experience a change in spin are said to be spin forbidden.

Laporte's rule: ? l = ± 1
d-d transitions for complexes which have a center of symmetry are forbidden - symmetry forbidden or Laporte forbidden.[2]

Charge transfer complexes do not experience d-d transitions. Thus, these rules do not apply and the absorptions are generally very intense.

For example, the classic example of a charge-transfer complex is that beween iodine and starch to form an intense purple color. This has wide-spread use as a rough screen for counterfeit currency. Unlike most paper, the paper used in US currency is not sized with starch. Thus, formation of this purple color on application of an iodine solution indicates a counterfeit.


It's from this link:
http://en.wikipedia.org/wiki/Charge_transfer_complex