Synthesis and molecular structure of niobocene trimethylacetate and its π-complex with diphenylacetylene

Niobocene trimethylacetate Cp 2 Nb(OOCCMe 3 ) (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 Cp 2 Nb(OOCCMe 3 )(π-Ph 2 C 2 ) · 0.5 C 6 H 6 (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 NbC distances of 2.39 and 2.44 A, respectively. The bisector plane of I contains the chelate trimethylacetate group (NbO bond lenghts 2.23 and 2.24 A) and that of II contains the coordinated tolane molecule and the oxygen atom of the terminal trimethylacetate ligand (NbO 2.16, NbC 2.18 and 2.19, CC 1.29 A, PhCC 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 CC triple bond in II are similar to those found in Cp(π-Ph 4 C 4 )Nb(CO)(π-Ph 2 C 2 ) studied earlier. The role played by the Nb III lone pair in I and II is discussed.