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(ORF)4]- (RF, 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.