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.

众所周知，多种均化过渡金属羰基络合物是7-12族金属的本体化合物。这些金属通常最多具有6个CO配体，与18个价电子形成配合物。相反，虽然价电子少的3-5族金属可形成高度配位的七羰基和八羰基络合物，尽管它们只能通过气相质谱法和/或基质分离谱法进行鉴定。现在，我们已经制备了铌和钽的七羰基阳离子，作为在室温下稳定的结晶盐。[M（CO）₇ ] ⁺（M = Nb或Ta）络合物是通过[M（CO）₆ ] ^–与2Ag ⁺ [Al（OR ^F）的氧化而形成的₄ ] ^–（R ^F，C（CF ₃）₃）在CO气氛下；它们的实验表征得到密度泛函理论计算的支持。还合成了其他不寻常的羰基化合物：两种包含84价电子簇阳离子[Ag ₆ {Nb（CO）₆ } ₄ ] ^2+的同构盐，钢琴凳配合物[（1,2-F ₂ C ₆ H ₄）M（CO）₄ ] ⁺和两个中性Ta ₂（CO）_12的多晶型物，带有长的，未支撑的Ta-Ta键。