Dear Hussein Ali Kadhim Kyhoiesh ,

When you are talking about spectra do you mean UV-Visible absorption spectra ? If it is this latter case, there are numerous publications, books dealing with ligand field theory and crystal field theory explaining well the necessary amount of energy (through light absorption) to trigger electron to occupied d-orbital to unoccupied d-orbital and explaining the numerous colours a metal-based complex could have. If you need more information I could lead you to some litterature.

In the case of FT-IR, I don't have enough knowledge (except if I take a look in my books!) to give you a desired answer. I just know that, in the case of metal-carbonyl complexes, (C=O) band in FT-IR is a good indicator to segregate between different complexes.

Dear Hussein Ali Kadhim Kyhoiesh,

In case of IR spectroscopy, the direction of the shift of the bands depends on the nature of the metal and ligands. For example, metal carbonyls displays IR absorption bands for CO at lower wavenumber than the free carbonyls. CO coordinates metals sinergistically (sigma donation and pi acceptance). In this case CO donates its electron to metals from a bonding type of molecular orbitals (HOMO), but accepts electron from filled metal d-orbital to the vacant antibonding type molecular orbital (LUMO). However both sigma donation and pi acceptance results a decrease in the C-O bond order which is consistent with the IR absorption band at lower frequency.

Dear Hussein, thank you for asking this interesting technical question. In the case of IR spectroscopy, it cannot be said in general that "all spectra of metal complexes are shifted to a lower wave number and reduced in intensity". This depends on various different factors such as the individual metal, the type of ligands, the oxidation state of the metal etc. As mentioned by Shafikul Islam the carbonyl ligand (CO) is a good example. Due to the possibility of pi-back-bonding the IR bands of CO ligands are normally shifted to lower wavenumbers as compared to free carbon monoxide (2143 cm-1). However, in the case of cationic metal carbonyl complexes, the carbonyl frequencies can be even higher than that of free CO. For a typical reference on this phenomenon please see this interesting research article (see attached pdf file):

Why Do Cationic Carbon Monoxide Complexes Have High C-O Stretching Force Constants and Short C-O Bonds? Electrostatic Effects, Not σ-Bonding

https://www.cup.uni-muenchen.de/ac/kluefers/homepage/L/kc1/carbonyl_sigma_1996.pdf

As mentioned by Corentin Lefebvre the IR spectroscopy of metal complexes cannot be fully outlined in a short answer. It would be helpful if someone offered a course on spectroscopic methods at your institution. For some very basic information please have a look at the following potentially useful link:

Fourier Transform Infrared Spectroscopy of Metal Ligand Complexes

https://cnx.org/contents/[email protected]:6oYLai6Q@1/Fourier-Transform-Infrared-Spectroscopy-of-Metal-Ligand-Complexes

Good luck with your work and best wishes, Frank Edelmann

One cannot anticipate any general trend in spectral features of complexes. Whether you take IR, uv-visible or NMR, it depends on the metal (3d, 4d or 5d) atom/ion, electronic configuration-that means, the number of d-electrons, the oxidation state, the coordination number and the type of ligands. One can observe either lower wave number or higher wave number. red shift or blue shift, shielding or deshielding in the respective spectral data . But it is always possible to give a satisfactory and convincing explanation about the outcome of spectral results and their features. If you are talking about the carbonyl stretching frequencies, If the metal is electron rich and coordinated by only sigma=donor or both sigma and pi-donors ligands, CO stretching frequency decreases dramatically because of the electronic population of CO anti-bonding orbitals. Like one can give explanation and understand the properties and hence reactivity and suitable applications.

best wishes

Balakrishna

Regarding the first question, it depends on the type of spectra. For UV-Vis spectra, when the metal complexes, the bands do not shift, the metal ligand and ligand metal charge transfer bands appear in the same area and with similar intensity as the intraligand bands. As for the d-d transitions, they appear in the same area, but here with less intensity.

On the other hand, in NMR spectra it depends on the magnetic characteristics of the metal ion. If it is a diamagnetic complex, it does not influence the NMR bands. On the other hand, if the complex is paramagnetic, it does affect the signals and these move both in high field and in low field, it depends on the magnetic tensor. The bands are usually wider, but the signal area does not change.

different factors

1-type of ligands, 2- oxidation state of the metal

-type of solvent

number of d-electrons and F-electrons