SYNTHESIS AND SPECTROSCOPIC PROPERTIES OF RE(R)(CO)(3)(ALPHA-DIIMINE) (R=ALKYL ALPHA-DIIMINE=R'-PYCA, R'-DAB) COMPLEXES - CRYSTAL-STRUCTURE OF RE(ME)(CO)(3)(PR-I-DAB)

The synthesis, structure and spectroscopic properties of several complexes of the type Re(R)(CO)(3)(alpha-diimine) (R = alkyl) are reported. The structure of Re(Me)(CO)(3)(Pr-i-DAB) has been determined by a single-crystal X-ray diffraction study. The molecule has a distorted octahedral geometry, with the three carbonyls in a fac-geometry. The complexes possess strong absorption bands in the visible region which are assigned to MLCT transitions with the aid of resonance Raman spectroscopy; the Raman spectra do not provide any evidence for a sigma(Re - R) --> pi*(alpha-diimine) transition within this absorption band. The W-vis spectrum of Re(Bz)(CO)(3)(Pr-i-DAB) (Bz = benzyl) shows a distinct shoulder at the low-energy side of the visible band, which is attributed to the interaction between the d pi(Re)-orbitals and the pi-orbitals of the benzyl group. The UV-photoelectron spectrum of Re(Me)(CO)(3)(Pr-i-DAB) shows that sigma(Re - Me) has a higher ionization potential than the d pi(Re)-orbitals, a result that is of importance for the interpretation of the photochemical behaviour of this complex.