VIBRATIONAL-SPECTRA, MOLECULAR-STRUCTURE AND ROTATIONAL-ISOMERISM IN STAIRCASE CARBONYL-COMPLEXES OF TRANSITION-METALS

Fourier transform infrared (FT-IR) spectra have been studied of ''stai rcase'' cyclopentadienyl complexes containing two or three metal carbo nyl fragments bound by the metal-carbon bond Cp(CO)(2)Fe-CpmMn(CO)(3) (I), Cp(CO)(2)Fe-CpmFe(CO)(2) CH(2)Ph(II), Cp(CO)(2)Fe-Cpm(CO)(2)Fe-Cp mMn(CO)(3)(III), Cp(CO)(2)Mo-Cpm(CO)(2)Fe-CpmMn(CO)(3)(IV), Cp(Co)(3)W -Cpm(CO)(2)Fe-CpmMn(CO)(3) (V), Cp(CO)(2)Fe-Cpm(CO)(2)Fe-BmCr(CO)(3) ( VI), Cr(CO)(3)Bm-CpmFe(CO)(2)CH(2)Ph (VII), where Cp = eta(5)-C5H5, Cp m = eta(1):eta(5)-C5H4, Bm = eta(1):eta(6)-C6H5, as well as model mono nuclear complexes Cp(CO)(2)FeCH(2)Ph (VII), CpMn(CO)(3) (IX), and (eta (6)-C6H6)Cr(CO)(3) (X). The spectra were interpreted on the basis of t he local symmetry of each metal carbonyl center. The vee positions are determined by the mutual electronic effects of each center. CpmM(CO)( n) groups are strong electron accepters and cause an increase in the u psilon(CO) of adjacent M(CO)(n) groups. Cp(CO)(n)M groups, being elect ron donors, lead to a decrease in the frequencies of neighbouring grou ps. Temperature dependent FT-IR spectra in pentane solutions and solid matrices were measured. Rotational isomers, formed due to rotation ab out the metal-carbon bond, were found in solutions and solid matrices. A molecular mechanics calculation for I proved the possibility of suc h rotation.