Hi Pretam Kumar , If we explain the scenario in the most simplest way, Compared to the free ligand, the ligand -metal complex will have a larger HOMO-LUMO gap upon formation of the complex. The simple reason for this is that the pi/lone pair electron will not be readily available upon formation of the metal ligand complex. This is just a simple explanation for the observed results. However, M-L complex optical properties can be more complex and interesting depending on the species.

Dear Pretam Kumar

Thank you very much for your nice technical question. However, I am little bit confused as you generalized the shift of absorption band of a ligand and its metal complexes. The absorption bands in UV-visible spectroscopy do not always show the blue shift, red shift is more common upon metal complexation. For example, the Schiff base type ligand derived from 5-Amino-4H-1,2,4-triazole-3-thiol and 3-hydroxy-4-methoxy benzaldehyde showed two absorption bands at 310 nm and 316 nm due to π-π* and n-π* electronic transitions respectively. However, all of its cobalt, copper, nickel, zinc and manganese complexes displayed a red shifts for both π-π* and n-π* electronic transitions. The article is shared as public full text article on RG. So you can easily find it from the following link. You can find more example in literature.

Article Preparation, spectral characterization and biological applic...

The shift of absorption band depends on the change in HOMO-LUMO energy gap. If metal complexation is associated with the decrease in HOMO-LUMO energy gap, the absorption band will surely shift to the longer wavelength (the red shift) and vice versa.

Good luck with your work and best wishes.

Dear Shafikul Islam

Thank you so much for your informative answer.

Can you please tell me the reason about the Red Shift in metal complexation. I shall be highly thankfull.

Best wishes,

Pretam

Dear Pretam Kumar

Thank you very much for your further interest in this topic. I think you know about the pi donor and pi acceptor ligands. Pi acceptor ligands show metal to ligand charge transfer (MLCT) effect which results a decrease in HOMO-LUMO gap. So for pi acceptor ligands, complexation is associated with a red shift in the absorption band. On the other hand for pi donor ligands displaying LMCT effect, the HOMO-LUMO energy gap is also decreased, but in this case the HOMO is formed with the greater percentage of the metal d-orbital. So one should consider the transition from the bonding type MO (this is formed with greater percentage of ligand orbital) to the LUMO (to compare the absorption bands with that of free ligand) for which the energy gap is increased and hence shows a blue shift in the absorption band. The attached figure may be helpful to you. For more information I would like suggest you an inorganic chemistry textbook "Inorganic Chemistry - G. L. Miessler, P. J. Fisher and D. A. Tarr, 5th edn. chapter 10". This book is freely available online. You can download this book by clicking on the following link.

https://celqusb.files.wordpress.com/2017/12/inorganic-chemistry-g-l-miessler-2014.pdf

Best wishes to you.

Dear Pretam and others,

if you have absorptions of pi-pi* and n-pi* character in your ligand, many things can happen upon coordination to a metal.

If the metal is electron-rich, metal-to-ligand charge transfer might occur in the complex. These MLCT bands are red-shifted compared to the pi-pi* bands and might obscure the (intrisically weak) n-pi* bands.

If the metal is electron-deficient (high oxidation state, high charge) ligand-to-metal charge transfer might occur. The LMCT bands appear also red-shifted to initial pi-pi* bands and might obscure the n-pi* bands.

At the same time, your ligand might be deprotonated upon coordination. Also this will change the absorption bands of the ligand.

But all this depends on your metal and its oxidation state, and the type of ligand (the type of chromophore).

So, without the spectrum and without any details on the complex and ligand, any interpretation is like stirring at the bottom of your coffee beaker - and reading the future out of it.

Pretam, if you like, show us the ligand and the metal and the spectra, then we can start discussing.