C-2 BUILDING-BLOCKS IN THE COORDINATION SPHERE OF ELECTRON-POOR TRANSITION-METALS - ASPECTS OF THE CHEMISTRY OF EARLY-TRANSITION-METAL CARBENOIDE COMPLEXES

Our understanding of the chemistry of the transition-metal-carbon sigm a bond is improved by investigations of the chemistry of alken-1-yl co mplexes of electron-poor transition metals. There is no other system k nown in which we can easily switch between the possible reaction pathw ays, depending on the nature of the metal, the ligands L and the alken -1-yl group. Only reductive elimination, alpha- and beta-H elimination reactions give high selectivity. alpha-Hydrogen elimination from CpT i-2(CH=CH2)R (Cp=eta-C5Me5) derivatives leads to the versatile titana -allene intermediate [CpTi-2=C=CH2]8. A wide range of cycloaddition p roducts of high thermal stability can be prepared using 8. In reaction s of 8 with copper and gold complexes, heterodinuclear mu-vinylidene c ompounds, CpTi-2(mu-C=CH2)(mu-X)M'L, are formed. Additionally the fir st examples of intermolecular carbene-carbene coupling reaction of a F ischer- and a Schrock-carbene ligand are reported by using the strong nucleophilic vinylidene fragment 8.