Zirconium(II)‐ and Hafnium(II)‐Assisted Reductive Coupling of Coordinated Carbonyl Groups Leading to Ketenylidene Complexes of Zirconium(IV) and Hafnium(IV)

The 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 ‐dialkylcarbamtes of the tetravalent metals [M(O 2 CNR 2 ) 4 ], the ketenylidene complexes [M 3 Cp 2 (μ 2 ‐CCO)‐(μ 3 ‐O)(O 2 CNR 2 ) 6 ] ( 1 a : MZr, REt; 1 b : MZr, R i Pr; 2 : MHf, R i Pr). 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 1 b shows that it consists of trinuclear molecules, with the three zirconium atoms held together by the bidentate C 2 O ligand, by the tridentate bridging oxide and by the bidentate carbamato groups. Labelling experiments with [ZrCp 2 ( 13 CO) 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 C 2 O ligand. The ketenylidene group of 1 b was readily removed and replaced by a bidentate μ‐oxo ligand of the same hapticity by reaction with carbon dioxide or acetone or by thermal decomposition. The resulting product [Zr 3 Cp 2 (μ‐O)(μ 3 ‐O)‐(O 2 CN i Pr 2 ) 6 ] ( 3 ) crystallizes in the same space group as 1 b and with similar cell constants and bond parameters.