CARBON-DIOXIDE COORDINATION CHEMISTRY .1. VIBRATIONAL STUDY OF TRANS-MO(CO2)2(PME3)4 AND FE(CO2)(PME3)4

This paper describes the spectroscopic characterization of carbon dioxide complexes and discusses the role of isotopic labeling for this purpose. In part 1, complete assignments of FTIR and Raman spectra are proposed for trans-Mo(CO2)2(PMe3)4 and Fe(CO2)(PMe3)4. Frequency shifts observed by means of isotopic enrichments with C-13 and/or O-18 are related to the coordination mode of the CO2 moiety. It appears that two vibrational modes are highly sensitive to the CO2 geometry in the complex: the symmetrical stretching mode v(CO), at about 1150 cm-1, and the out-of-the-plane bending mode gamma(C = O), located around 550 cm-1. Normal coordinate analysis allows us to conclude that the CO2 moiety is bonded in a side-on manner in trans-Mo-(CO2)2(PMe3)4, whereas the bonding is intermediate between an eta-1-C and a side-on arrangement in Fe(CO2)(PMe3)4.