Synthesis and characterization of crystalline niobium and tantalum carbonyl complexes at room temperature

A variety of homoleptic transition metal carbonyl complexes are known as bulk compounds for group 7-12 metals. These metals typically feature a maximum of 6 CO ligands to form complexes with 18 valence electrons. In contrast, group 3-5 metals, with fewer valence electrons, have been shown to form highly coordinated heptacarbonyl and octacarbonyl complexes-although they were only identified by gas-phase mass spectrometry and/or matrix isolation spectroscopy work. Now we have prepared heptacarbonyl cations of niobium and tantalum as crystalline salts that are stable at room temperature. The [M(CO)₇]⁺ (M = Nb or Ta) complexes were formed by the oxidation of [M(CO)₆]^- with 2Ag⁺[Al(OR^F)₄]^- (R^F, C(CF₃)₃) under a CO atmosphere; their experimental characterization was supported by density functional theory calculations. Other unusual carbonyl compounds were also synthesized: two isostructural salts that contained the 84-valence-electron cluster cation [Ag₆{Nb(CO)₆}₄]²⁺, the piano-stool complexes [(1,2-F₂C₆H₄)M(CO)₄]⁺ and two polymorphs of neutral Ta₂(CO)₁₂ with a long, unsupported Ta-Ta bond.