Vibrational spectra of some monosubstituted-π-arene tricarbonyl-chromium complexes and of methyl benzoate

Abstract

New Raman polarisation data for Cr(CO)₃(π-arene), where (arene)= aniline, anisole, and methyl benzoate, have allowed essentially complete assignments of the spectra of the π-arenes in these compounds. A new assignment is also given for methyl benzoate and corrections are made to published work on Cr(CO)₃(π-C₆D₆) and Cr(CO)₃(π-mesitylene) which again emphasises the failure of the ‘local’ symmetry concept and the need to use full factor-group analysis. By comparison with the established frequencies of benzene in its first electronic excited state it is shown that many arene modes shift in the direction expected, if there is partial population of the π* orbitals due to back-donation from the metal, but others, notably π-(C–H) modes, shift in the opposite sense owing to kinematic coupling.