Highly Reduced Organometallics. 16. Synthesis, Isolation, and Characterization of the Tricarbonylcyclopentadlenylmetalate(2-) Anions of Niobium and Tantalum, C5H5M(CO)₃^2-

Treatment of C₅H₆M(CO)₄ (M = Nb, Ta) with sodium in liquid ammonia at-78 °C generates the corresponding disodium salts Na₂[C₆H₆M(CO)₃], which have been spectroscopically identified. Cesium iodide reacts with Na₂[C5H₆M(CO)₃] in liquid ammonia to provide 70-80% isolated yields of pale orange, air-sensitive solids that analyze satisfactorily as the unsolvated dicesium salts Cs₂[C₆H₆M(CO)₃]. These are the first isolated salts containing the tricarbonylcyclopentadienylmetalate dianions of niobium and tantalum. Reactions of C₅H₅M(CO)₃^2- with CH₃CN, Et₄N⁺, or other weak Bronsted acids or Ph₃SnCl provide, after cation exchange, 40-70% isolated yields of [Et₄N] [C₆H₆M(CO)₃H] or [Et₄N][C₅H₅M(CO)₃SnPh₃], which are characterized by elemental analyses and ¹H and NMR spectra.