Zirconium(II)- and hafnium(II)- assisted reductive coupling of coordinated carbonyl groups leading to ketenylidene complexes of zirconium(IV) and hafnium(IV)

he biscyclopentadienyldicarbonyl derivatives of zirconium(II) and hafnium(II) ([MCp(2)(CO)(2)]; M = Zr, Hf) promote the reductive coupling of coordinated carbon monoxide to give, in the presence of N,N-dialkylcarbamates of the tetravalent metals [M(O(2)CNR(2))(4)], the ketenylidene 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 the isolated zirconium complexes are as high as 60%, while that of the hafnium derivative is 40%. The X-ray crystal structure analysis of 1b shows that it consists of trinuclear molecules, with the three zirconimu atoms held together by the bidentate C2O ligand, by the tridentate bridging oxide and by the bidentate carbamato groups, Labelling experiments with [ZrCp(2)((CO)-C-13)(2)] show that the ketenylidene ligand originates from the coordinated CO groups. These ketenylidene complexes, which are rare examples of compounds containing a CCO but no CO ligand, are characterized by an intense IR band at about 2015 cm(-1), associated with the bridging C2O ligand. The ketenylidene group of 1b was readily removed and replaced by a bidentate mu-oxo ligand of the same hapticity by reaction with carbon dioxide or acetone or by thermal decomposition, 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 cell constants and bond parameters.