Stabilization of a planar-tetracoordinate carbon center in an organometallic complex containing both a zirconocene and a hafnocene moiety

The methylhafnocene cation 2a reacts with bispropynylzirconocene to give a single regioisomeric planar-tetracoordinate carbon compound (4a) that contains the hafnium atom singly-bonded and the zirconium center eta(2)-coordinated to the bridging mu-(eta(1)-C:eta(2)-C,C-2-butyne) ligand. However, the reaction between the methylzirconocene cation 2b with bis(phenylacetylide)hafnocene (3b) gives the pure regioisomer 4b with an opposite relative arrangement of the two group 4 transition metals (the methyl is at the planar-tetracoordinate carbon atom C2). Complex 4a was characterized by an X-ray crystal structure analysis.