Teaching bismuth to make and break bonds: One major reason why transition metals are good catalysts is that they can shuttle between oxidation states. This flexibility lets them slide in and out of chemical bonds; so, for instance, they can snip a bond between carbon and boron and then stitch a carbonfluorine bond in its place. Planas et al. now report that bismuth can also orchestrate such bond-swapping events. They implement a fully catalytic cycle for fluorination of aryl boronates, in which bismuth hops between its +3 and +5 oxidation states. Science , this issue p. 313

Silicon marries a chiral counterion: Acid is among the oldest and most versatile of chemical catalysts, but its symmetrical protons can't guide reactions to favor a product over its mirror image. Chemists have resolved this shortcoming through the use of chiral conjugate bases. While the proton activates the substrate, the nearby counterion asymmetrically biases the space around it. Gatzenmeier et al. extend this approach to Lewis acid catalysis by silyl cations, which can activate a variety of substrates in complementary fashion to protons (see the Perspective by Dumoulin and Masson). By pairing these silyl groups with chiral carbon-based anions, they achieve highly enantioselective catalysis of Diels-Alder reactions. Science , this issue p. 949 ; see also p. 918

Herein we present the catalytic activation of N₂O at a Bi^I⇄Bi^III redox platform. The activation of such a kinetically inert molecule was achieved by the use of bismuthinidene catalysts, aided by HBpin as reducing agent. The protocol features remarkably mild conditions (25 °C, 1 bar N₂O), together with high turnover numbers (TON, up to 6700) and turnover frequencies (TOF). Analysis of the elementary steps enabled structural characterization of catalytically relevant intermediates after O-insertion, namely a rare arylbismuth oxo dimer and a unique monomeric arylbismuth hydroxide. This protocol represents a distinctive example of a main-group redox cycling for the catalytic activation of N₂O.

Insights into the mechanism of the unusual trans‐hydrogenation of internal alkynes catalyzed by {Cp*Ru} complexes were gained by para‐hydrogen (p‐H₂) induced polarization (PHIP) transfer NMR spectroscopy. It was found that the productive trans‐reduction competes with a pathway in which both H atoms of H₂ are delivered to a single alkyne C atom of the substrate while the second alkyne C atom is converted into a metal carbene. This “geminal hydrogenation” mode seems unprecedented; it was independently confirmed by the isolation and structural characterization of a ruthenium carbene complex stabilized by secondary inter‐ligand interactions. A detailed DFT study shows that the trans alkene and the carbene complex originate from a common metallacyclopropene intermediate. Furthermore, the computational analysis and the PHIP NMR data concur in that the metal carbene is the major gateway to olefin isomerization and over‐reduction, which frequently interfere with regular alkyne trans‐hydrogenation.

Large spin-orbit coupling (SOC) is an intrinsic property of the heavy elements that directly affects the electronic structures of the compounds. In this work, we report the synthesis and characterization of a monocoordinate bismuthinidene that features a rigid and bulky ligand. All magnetic measurements [superconducting quantum interference device (SQUID), nuclear magnetic resonance (NMR)] point to a diamagnetic compound. However, multiconfigurational quantum chemical calculations predict the ground state of the compound to be dominated (76%) by a spin triplet. The apparent diamagnetism is explained by an extremely large SOC-induced positive zero-field splitting of more than 4500 wavenumbers that leaves the M _S = 0 magnetic sublevel thermally isolated in the electronic ground state.