ETA-2 VERSUS ETA-1 COORDINATION OF ALDEHYDES AND KETONES IN ORGANOMETALLIC COMPLEXES - A SEMIEMPIRICAL THEORETICAL-STUDY

The eta-1 and eta-2 coordinations of aldehydes and ketones on several types of organometallic fragments have been compared on the basis of an extended Huckel molecular orbital analysis. The electronic description of the interaction between the organic ligand and the metallic part leads to the distinction between stabilizing two electron interactions and destabilizing four electron ones. The stabilizing interactions concern the frontier orbitals, oxygen lone pair with the LUMO of the metallic fragment for the eta-1, mode, and pi*CO with occupied d for the eta-2 mode and always favor the eta-2 coordination. However, four electron interactions between low-lying orbitals play an important role in the determination of the preferred structure. These interactions result from an indirect coupling through the metal center between occupied orbitals of the organic molecule and those of other ligands on the complex. For overlap reasons these ligand-ligand 'through-bond" interactions are stronger for the eta-2 mode, thus making this structure less energetically favored. The balance between these two conflicting electronic effects is described for various metals, ligand environments, and substituents on the organic molecule. On this basis, the behavior of d10 ML2, d8 ML3, d6 ML5, and CpML2 type fragments is detailed. We show how the preferred coordination can be changed for a given type of organometallic fragment by a modification of ligands or substituents.