Breaking and Making of Carbon–Carbon Bonds by Lanthanides and Third‐Row Transition Metals

Carbon‐atom extrusion from the ipso ‐position of a halobenzene ring (C 6 H 5 X; X=F, Cl, Br, I) and its coupling with a methylene ligand to produce acetylene is not confined to [LaCH 2 ] + ; also, the third‐row transition‐metal complexes [MCH 2 ] + , M=Hf, Ta, W, Re, and Os, bring about this unusual transformation. However, substrates with substituents X=CN, NO 2 , OCH 3 , and CF 3 are either not reactive at all or give rise to different products when reacted with [LaCH 2 ] + . In the thermal gas‐phase processes of atomic Ln + with C 7 H 7 Cl substrates, only those lanthanides with a promotion energy small enough to attain a 4f n 5d 1 6s 1 configuration are reactive and form both [LnCl] + and [LnC 5 H 5 Cl] + . Branching ratios and the reaction efficiencies of the various processes seem to correlate with molecular properties, like the bond‐dissociation energies of the C−X or M + −X bonds or the promotion energies of lanthanides.