SANDWICH AND TRIPLE-DECKER COMPLEXES OF COBALT AND RHODIUM INVOLVING THE BOROLE LIGAND - PHOTOELECTRON SPECTROSCOPIC INVESTIGATIONS AND AB-INITIO CI CALCULATIONS ON THE PHOTOELECTRON-SPECTRA

The photoelectron (PE) spectra of four sandwich complexes involving the methylborole ligand C4H4BCH3 are reported. Two of those molecules, namely (eta5-C4H4BCH3)M(C5H5) (M = Co (1a), Rh (2a), are 18-electron complexes, the other ones, namely (mu-eta5-C4H4BCH3)[M(eta5-C4H4BCH3)]2 (M = Co (3a), Rh (4a)), being triple-decker complexes with 30 electrons. The reported spectra are interpreted on the basis of ab initio configuration interaction (CI) calculations, carried out on model complexes where methylborole ligands have been replaced by C4H4BH cycles (lb-4b), and on the basis of Green's function formalism adapted to an improved INDO procedure (1a, 3a). The ab initio CI calculations showed that the assignment of the two first band systems of the PE spectra is interchanged when cobalt is replaced by rhodium in isoelectronic complexes. This is a consequence of the relatively small relaxation effects and large ligand field splittings that are characteristic of the diffuse d shell of second-row transition metals. In the case of la and 2a our assignment is supported by intensity arguments from He I and He II PE spectra. The INDO results on 1a and 3a agree with the ab initio results.