SYNTHESIS AND MOLECULAR-STRUCTURE OF NIOBOCENE TRIMETHYLACETATE AND ITS PI-COMPLEX WITH DIPHENYLACETYLENE

Niobocene trimethylacetate Cp2Nb(OOCCMe3) (I) does not react with usual n-donors (pyridine and triphenylphosphine), but readily adds a π-acceptor molecule of diphenylacetylene (tolane) in benzene to form Cp2Nb(OOCCMe3)(π-Ph2C2) · 0.5 C6H6 (II). The structures of the diamagnetic complexes I and II have been determined by an X-ray diffraction study. These molecules represent wedge-like sandwiches wit dihedral angles between cyclopentadienyl ligands equal to 44.4 and 50.7°, and average NbC distances of 2.39 and 2.44 Å, respectively. The bisector plane of I contains the chelate trimethylacetate group (NbO bond lenghts 2.23 and 2.24 Å) and that of II contains the coordinated tolane molecule and the oxygen atom of the terminal trimethylacetate ligand (NbO 2.16, NbC 2.18 and 2.19, CC 1.29 Å, PhCC angles 141 and 146°). An unusually large splitting of OCO stretching frequencies is observed in the IR spectrum of I (1652-1305 = 347 cm-1). Structural characteristics of the coordinated CC triple bond in II are similar to those found in Cp(π-Ph4C4)Nb(CO)(π-Ph2C2) studied earlier. The role played by the NbIII lone pair in I and II is discussed. © 1979.