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(CF 3 ) 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 [Ag 6 {Nb(CO) 6 } 4 ] 2+ , the piano-stool complexes [(1,2-F 2 C 6 H 4 )M(CO) 4 ] + and two polymorphs of neutral Ta 2 (CO) 12 with a long, unsupported Ta–Ta bond.