A Facile Atmospheric Pressure Synthesis of the Hexacarbonylmetalate Ions, M(CO)₆, of Niobium and Tantalum

Treatment of the pentachlorides of niobium and tantalum with 6 equiv of alkali-metal naphthalenide or 1-methylnaphthalenide in DME at -60 to -80 °C provides thermally unstable red-brown intermediates that may be M(C10H8)2- or M(1-MeC10H7)2_ (M = Nb, Ta). These species readily react with CO at atmospheric pressure at -60 °C to provide 30-54% yields of the corresponding M(CO)6”, isolated as Et4N+, n-Bu4N+, and Na(diglyme)2+ salts. This two-step reductive carbonylation procedure is the first reported atmospheric pressure synthesis of the hexacarbonylmetalate(l-) ions of niobium and tantalum. The synthesis and properties of unsolvated NaTa(CO)6 are also described.