ZIRCONIUM(II)-ASSISTED AND HAFNIUM(II)-ASSISTED REDUCTIVE COUPLING OFCOORDINATED CARBONYL GROUPS LEADING TO KETENYLIDENE COMPLEXES OF ZIRCONIUM(IV) AND HAFNIUM(IV)

The biscyclopentadienyldicarbonyl derivatives of zirconium(II) and haf nium(II) ([MCp(2)(CO)(2)]; M = Zr, Hf) promote the reductive coupling of coordinated carbon monoxide to give, in the presence of N,N-dialkyl carbamates of the tetravalent metals [M(O(2)CNR(2))(4)], the ketenylid ene complexes [M(3)Cp(2)(mu(2)-CCO)(mu(3)-O)(O(2)CNR(2))(6)] (1a: M = Zr, R = Et; 1b: M = Zr, R = iPr; 2: M = Hf, R = iPr). The yields of th e isolated zirconium complexes are as high as 60%, while that of the h afnium derivative is 40%. The X-ray crystal structure analysis of 1b s hows that it consists of trinuclear molecules, with the three zirconim u atoms held together by the bidentate C2O ligand, by the tridentate b ridging oxide and by the bidentate carbamato groups, Labelling experim ents with [ZrCp(2)((CO)-C-13)(2)] show that the ketenylidene ligand or iginates from the coordinated CO groups. These ketenylidene complexes, which are rare examples of compounds containing a CCO but no CO ligan d, are characterized by an intense IR band at about 2015 cm(-1), assoc iated with the bridging C2O ligand. The ketenylidene group of 1b was r eadily removed and replaced by a bidentate mu-oxo ligand of the same h apticity by reaction with carbon dioxide or acetone or by thermal deco mposition, The resulting product [Zr(3)Cp(2)(mu-O)(mu(3)-O)(O2CNiPr2)( 6)] (3) crystallizes in the same space group as 1b and with similar ce ll constants and bond parameters.