Transition metal carbonyls vibrational frequencies

The period under review has seen a small, but apparently real, decrease in the annual number of publications in the field of the vibrational spectroscopy of transition metal carbonyls. Perhaps more important, and not unrelated, has been the change in perspective of the subject over the last few years. Although it continues to be widely used, the emphasis has moved from the simple method of v(CO) vibrational analysis first proposed by Cotton and Kraihanzel2 which itself is derived from an earlier model4 to more accurate analyses. One of the attractions of the Cotton-Kraihanzel model is its economy of parameters, making it appropriate if under-determination is to be avoided. Two developments have changed this situation. Firstly, the widespread availability of Raman facilities has made observable frequencies which previously were either only indirectly or uncertainly available. Not unfrequently, however, these additional Raman data have been obtained from studies on crystalline samples, a procedure which, in view of the additional spectral features which can occur with crystalline solids (vide infra), must be regarded as questionable. The second source of new information has been studies on isotopically-labelled species. 

Bond stretching frequency (measured by vibrational spectroscopy such IR spectroscopy, etc.) is related to stretching force constant and increases with increase in bond strengths. This study is quite useful for transition metal carbonyls. 

The present paper summarises the findings of our studies of force fields and vibrational frequencies of transition metal complexes. We discuss transition-metal-carbonyl complexes and complexes with small aromatic rings as ligands in detail. Benzene has an important role in this investigation as a Ugand, as well as an excellent benchmark test molecule. Accordingly, we also review the findings of our benzene force field in this report. 

We have reproduced the vibrational frequencies of benzene, transition metal carbonyls and transition metal complexes of benzene and cyclopentadiene fairly accurately. Our calculations indicate some ambiguities in the original empirical frequency assignments for ferrocene and dibenzene-chromium, for which we suggest alternatives. Our calculations confirm the frequency assignments for BzCr(CO)3. 

Zhou, M., Andrews, L., Bauschlicher, C. (2001). Spectroscopic and theoretical investigations of vibrational frequencies in binary unsaturated transition-metal carbonyl cations, neutrals, and anions. Chemical Reviews, 101,1931-1961. 

An indirect yet powerful tool to monitor the metal adsorption sites is to study CO adsorption. CO is widely used as a probe molecule because of its low reactivity and quite sensitive vibrational frequency Small changes in the substrate electronic structure reflect into measurable changes in CO stretching frequency. The study of CO desorption temperature from a metal covered MgO film and of the IR bands associated to the formation of metal-carbonyl complexes provides an useful tool to identify the surface sites involved in the stabilization of the M-CO species [31]. However, for an atomistic view of the sites involved, it is essential to combine the experimental evidences with the results of ab initio calculations. Recently, this has been done for a series of transition metal atoms, Rh, Pd, and Ag, all belonging to the second transition series but characterized by quite different valence structures (Rh, d Pd, di° Ag, d si) [211,212]. 

As an indication of the importance of charge on the energy levels of molecules, we look at the variation in carbonyl stretching vibrational frequencies in the series Mn(CO)6, Cr(CO)6, and V(CO)6. All arc low spin isoelectronic molecules. The orbital details of the attachment of the carbon mono.xidc ligand to a transition metal are reserved for Section 15.1, but an important part is played by the acceptor behavior of the carbonyl 77 0 (8.12). The more important donation from... 

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