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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.

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Fig. 1: Overview to the syntheses of 1, 2, 3, 4 and 5.
Fig. 2: Single crystal X-ray structure determinations (scXRD) of 1, 2, 3, 4 and 5.
Fig. 3: Vibrational spectroscopy analysis of 2.
Fig. 4: Synthesis and molecular structure of 6.

Data availability

Atomic coordinates and structure factors for the crystal structure of 16 are deposited at the Cambridge Crystallographic Data Centre (CCDC) under the accession codes 1909275 (1), 1909276 (2), 1909277 (3a), 1909279 (3b), 1909278 (4), 1909449 (5), 1909450 (6a) and 1909274 (6b); copies of the data can be obtained from All the other data generated or analysed during this study are included in this published article (and its Supplementary Information files), and are available from the corresponding authors on reasonable request.


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Author information




I.K. supervised the project. W.U. designed and performed the experiments and all the characterizations. M.S. and D.H. carried out the general quantum chemical calculations. W.U. and D.K. performed the single-crystal determination and refinement. All the authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to I. Krossing.

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Competing interests

The authors declare no competing interests.

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Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Methods, syntheses of all compounds, vibrational, NMR and UV–vis analysis, crystallographic data and computational details.

Crystallographic Data

CIF for compound 1; CCDC reference: 1909275.

Crystallographic Data

CIF for compound 2; CCDC reference: 1909276.

Crystallographic Data

CIF for compound 3a; CCDC reference: 1909277.

Crystallographic Data

CIF for compound 3b; CCDC reference: 1909279.

Crystallographic Data

CIF for compound 4; CCDC reference: 1909278.

Crystallographic Data

CIF for compound 5; CCDC reference: 1909449.

Crystallographic Data

CIF for compound 6a; CCDC reference: 1909450.

Crystallographic Data

CIF for compound 6b; CCDC reference: 1909274.

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Unkrig, W., Schmitt, M., Kratzert, D. et al. Synthesis and characterization of crystalline niobium and tantalum carbonyl complexes at room temperature. Nat. Chem. 12, 647–653 (2020).

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