The occurrence of aromaticity in organic molecules is widely accepted, but its occurrence in purely metallic systems is less widespread. Molecules comprising only metal atoms (M) are known to be able to exhibit aromatic behaviour, sustaining ring currents inside an external magnetic field along M–M connection axes (σ-aromaticity) or above and below the plane (π-aromaticity) for cyclic or cage-type compounds. However, all-metal compounds provide an extension of the electrons’ mobility also in other directions. Here, we show that regular {Bi6} prisms exhibit a non-localizable molecular orbital of f-type symmetry and generate a strong ring current that leads to a behaviour referred to as φ-aromaticity. The experimentally observed heterometallic cluster [{CpRu}3Bi6]–, based on a regular prismatic {Bi6} unit, displays aromatic behaviour; according to quantum chemical calculations, the corresponding hypothetical Bi62− prism shows a similar behaviour. By contrast, [{(cod)Ir}3Bi6] features a distorted Bi6 moiety that inhibits φ-aromaticity.

Key challenges in modern synthetic chemistry include the design of reliable, selective, and more sustainable synthetic methods, as well as the development of promising candidates for new materials. Molecular bismuth compounds offer valuable opportunities as they show an intriguing spectrum of properties that is yet to be fully exploited: a soft character, a rich coordination chemistry, the availability of a broad variety of oxidation states (at least + V to À I) and formal charges (at least + 3 to À 3) at the Bi atoms, and reversible switching between multiple oxidation states. All this is paired with the status of a non-precious (semiÀ )metal of good availability and a tendency towards low toxicity. Recent findings show that some of these properties only come into reach, or can be substantially optimized, when charged compounds are specifically addressed. In this review, essential contributions to the synthesis, analyses, and utilization of ionic bismuth compounds are highlighted.

Experimental and theoretical studies are reported of the first two-coordinated Si 0 -isocyanide compound (SIDipp)Si CN−Ar Mes (1: SIDipp (NHC) = C[N(Dipp)CH 2 ] 2 , Ar Mes = 2,6dimesitylphenyl), supported by an N-heterocyclic carbene (NHC). A Si atom economic two-step synthesis of 1 involves a 2e reduction of the isocyanide-stabilized silyliumylidene salt [SiBr(CNAr Mes )-(SIDipp)][B(Ar F ) 4 ] (2[B(Ar F ) 4 ], Ar F = B(C 6 H 3 -3,5-(CF 3 ) 2 ) 4 ) with KC 8 . 2[B(Ar F ) 4 ] was obtained from SiBr 2 (SIDipp) after bromide abstraction with an equimolar mixture of Na[B(Ar F ) 4 ] and Ar Mes NC. Exact adherence to the stoichiometry is crucial in the latter reaction, since 2[B(Ar F ) 4 ] reacts with SiBr 2 (SIDipp) via isocyanide exchange to afford the disilicon(II) salt [Si 2 Br 3 (SIDipp) 2 )][B(Ar F ) 4 ] (3[B(Ar F ) 4 ]), the reaction leading to an equilibrium that favors 3[B(Ar F ) 4 ] (K eq (298 K) = 10.6, ΔH°= −10.6 kJ mol −1 ; ΔS°= −16.0 J mol −1 K −1 ). 3[B(Ar F ) 4 ] was obtained selectively from the 2:1 reaction of SiBr 2 (SIDipp) with Na[B(Ar F ) 4 ] and fully characterized. Detailed studies of 1 reveal an intriguing structure featuring a planar C NHC −Si−C−N skeleton with a V-shaped geometry at the dicoordinated Si 0 center, a slightly bent SiCN core, a C NHC −Si−C CNR 3c-2e out of plane π-bond (HOMO), and an anticlinal conformation of the SIDipp and Ar Mes substituents leading to axial chirality and the presence of two enantiomers, (R a )-1 and (S a )-1. Compound 1 displays structural dynamics in solution, rapidly interconverting the enantiomers. The silacumulene 1 is a potent Si(SIDipp) transfer agent as demonstrated by the synthesis and full characterization of the NHC-supported germasilyne (Z)-(SIDipp)(Cl)SiGeAr Mes (4) from 1 and Ge(Ar Mes )Cl.

First examples of coordinatively unsaturated, homoleptic azido complexes of low-valent group 14 elements are reported. A simple strategy uses low-valent precursors, ionic azide transfer reagents and bulky cations to obtain salt-like compounds containing E(N3)3(-) of Ge(II)/Sn(II) which are fully characterised, including XRD. Remarkably, these compounds are kinetically stable at r.t. and isolable in sub-gram quantities.