Highly Reduced Cyanido Metalates and Acetonitriltriides

• Applicants: Dr. Peter Höhn; Professor Dr. Michael Ruck
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 422042965

The cyanide ligands CN(–) in classical cyanidometalates are pure sigma-donors with bond lengths d(C–N) = 113–116 pm and stretching frequencies v(CN) = 2000–2200 cm(–1). In contrast, our new highly reduced cyanometalates, e.g. Ba3[Co(CN)3], exhibit significantly longer distances (120–126 pm) and reduced frequencies (1480–1729 cm(–1)). This indicates a severe weakening of the C–N bond. Electrons are shifted from the metal center towards the antibonding orbital of the CN bond by M–C pi bonding. These electron-rich complexes are characterized by very low or even negative oxidation states on the central metal atom as well as by reduced CN(x–) ligands with x > 1. Such cyanometalates were obtained with metals of groups 8 and 9, as well as with Ni in the previously known [(NC)NiN](4–). The isoelectronic trigonal planar complex anions [M(CN)3](6–) with M = Co, Rh, Ir and [M(CN)3](7–) with M = Fe, Ru share a common bonding situation including a metal d10 configuration as well as reduced CN(1.67–) ligands. Metalates [M(CN)3](7–) with M = Fe, Ru exhibit even lower stretching frequencies indicating weaker CN bonds due to the higher overall charge of the complex anion and thus stronger M–C π interactions. The properties of the tetrahedral cyanoferrate [Fe(CN)4](6−) resemble those of the isoelectronic carbonylferrate [Fe(CO)4](2−). In order to gain more insight into the stabilization conditions of highly reduced cyanidometalates, the project will concentrate on those transition metals that form highly reduced carbonyl compounds but no cyanometalate analogs so far, i.e. Cr, Mn, Cu. The reducing metals shall be extended to Mg, Ca, Sr, Ba, and Li allowing for specific structural arrangements and electronic situations needed for stabilizing the respective cyanidocomplexes. Moreover, the discovery of the electron-rich acetonitriltriide anion CCN(3–) in the nitridotantalate Ba5[TaN4][C2N] opens new chemical perspectives: Beyond highly reduced cyanidometalates, electron-rich molecular anions can also be stabilized in an alkaline-earth-rich nidridometalate structures. To validate this approach, nitridometalates containing the CCN(3–) anion and other known (such as nitridoborates BN2(3–) and allenides C3(4–) or hitherto unknown highly charged molecular anions shall be explored. Access to these new classes of compounds is provided by different preparation methods based on reactions in alkaline fluxes, metathesis reactions, or soft-chemistry techniques. The comprehensive characterization of the samples should further improve the understanding of these classes of compounds and possibly uncover interesting chemical and physical properties.

• DFG Programme: Research Grants