STRUCTURE AND REACTIVITY OF CP(2)ZR(ETA(2)-ME(2)SI=N(T)BU)(CO) - AN UNUSUAL SILANIMINE CARBONYL COMPLEX WITH EXTENSIVE SIGMA-PI BACK BONDING

The carbonyl silanimine complex Cp(2)Zr(eta(2)-Me(2)Si=N(t)Bu) (CO) (2) has been prepared by ligand substitution on the phosphine derivative, Cp(2)Zr(eta(2)-Me(2)Si=N (t)Bu)(PMe(3)) (1) and by generation of the alkyl derivative Cp(2)Zr(CH(2)SiMe(3)) (N(t)BuSiMe(2)H) under a carbon monoxide atmosphere. Most aspects of the spectroscopic properties and structural parameters are consistent with the formulation of 2 as a zirconium (IV) metallacycle, containing a Zr-Si-N three-membered ring and a linear, terminal carbonyl ligand. However, the carbonyl stretching frequency for 2 is observed at 1797 cm(-1) in the infrared, much lower than expected for a zirconium (IV) complex, and indeed, lower than any other zirconocene carbonyl. ZINDO molecular orbital calculations indicate that the origin of this anomalously low stretching band is donation of electron density into the CO pi* orbital directly from an orbital on the adjacent silicon center, i.e. sigma-pi* back-bonding. Consistent with this picture, the formally non-bonding Si...C distance in 2 (2.347(7) Angstrom) is much longer than a typical single bond (Ca 1.87 Angstrom), but in the range observed for delocalized organic pi-ligands bonded to silicon. Although the bond between the silicon and carbonyl ligand is not fully formed in the ground state of 2, thermolysis in the presence of PMe(3) yields the five-membered metallacycle cyclo-Cp(2)Zr[OC(=PMe(3)) SiMe(2)N(t)Bu] (3), resulting from CO insertion into the Zr-Si bond and formation of a cyclic silaacyl, followed by attack of phosphine at the electrophilic acyl carbon.