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The coordination of an element–element σ bond to a transition metal (TM) is both a fundamentally intriguing binding mode and of critical importance to metal-mediated bond activation mechanisms and catalysis, particularly the hotly contested field of C–H activation. TM σ complexes of dihydrogen (i.e., H–H) and silanes (H–SiR3) have been extensively studied, the latter being of interest as models for the (generally unstable and unisolable) σ complexes of alkanes (i.e., H–CR3). TM σ complexes of hydroboranes and hydroborates (i.e., H–BR2, H–BR3, (H−)2BR2) are somewhat less well studied but similarly have relevance to catalytic borylation reactions that are of high current interest to organic synthesis. Our two research groups have made significant contributions to elaborating the family of σ-borane/-borate complexes using two distinct approaches: while the Ghosh group generally starts from hydrogen-rich tetracoordinate boron species such as borates, the Braunschweig group starts from hypovalent and/or hypocoordinate boron building blocks. Through these two approaches, a wide range of species containing one or two σ-bound B–H ligands have been prepared, some with additional chelating donor sites. Over the past 2 years, the body of work on σ-borane/-borate complexes from our two research groups has significantly expanded, with a combined nine published articles in 2019–2020 alone. Very recent work from the Braunschweig group has led to the synthesis of the first bis(σ)-borane complexes of group 6 metals, as well as the synthesis of a series of novel bis(σ)-borane and bis(σ)-borate complexes of ruthenium and iridium, the former being useful precursors for pentacoordinate borylene complexes of Ru. Recent work from the Ghosh group has uncovered a remarkable diversity of structures with σ(B–H)-bound ligands from the combination of borohydrides and nitrogen/chalcogen-containing groups and heterocycles. These reactions, while in some cases producing conventional scorpionate-type chelating products, more frequently undergo fascinating rearrangements with unpredictable outcomes. This Account aims to highlight this recent acceleration of research progress in this area, particularly the distinct but related approaches of—and complexes produced by—our two research groups, in addition to relevant works from other groups where appropriate.

A range of mono- and bis-adducts of beryllium dichloride with cyclic (alkyl)(amino)carbenes (CAAC), cyclic diamidocarbene (DAC), and carbodiphosphorane (CDP) are prepared, and their reactions with nucleophiles are studied. Salt elimination with Yamashita and Nozaki’s lithium diazaborolide reagent led to the isolation of an unsymmetrical beryllium diazaborolyl complex and a base-stabilized diazaborolyl beryllium chloride. From structural and spectroscopic analyses, the Be–B bonding in these compounds was determined to be polar covalent in character. In addition, the nucleophilic addition of magnesium anthracenediyl to one of the adducts resulted in the isolation of an interesting tetracyclic beryllium-bridged molecule.

A series of novel isocloso-diiridaboranes [(Cp*Ir)2B6H6], 1, 2; [1,7-(Cp*Ir)2B8H8], 4; [1,4-(Cp*Ir)2B8H8], 5; [(Cp*Ir)2B9H9], 8; isonido-[(Cp*Ir)2B7H7], 3; and 10-vertex cluster [5,7-(Cp*Ir)2B8H12], 6(Cp* = η5-C5Me5) have been isolated and structurally characterized from the pyrolysis of [Cp*IrCl2]2and BH3·thf. On the other hand, the corresponding rhodium system afforded 10- and 11-vertices clusters [5-(Cp*Rh)B9H13)], 7, and [(Cp*Rh)2B9H9], 9, respectively. Clusters 1and 2are topological isomers. The geometry of 1is dodecahedral, similar to that of its parent borane [B8H8]2–, in which two of the [BH] vertices are replaced by two [Cp*Ir] fragments. The geometry of 2can be derived from a nine-vertex tricapped trigonal prism by removing one of the capped vertices. Compounds 4and 5are 10-vertex isoclosoclusters based on a 10-vertex bicapped square antiprism structure. The only difference between them is the presence of a metal–metal bond in 5. The solid-state structures of 8and 9attain an 11-vertex geometry in which a unique six-membered B6H6moiety is bonded to the metal center. In addition, quantum-chemical calculations have been used to provide further insight into the electronic structure and stability of the clusters. All the compounds have been characterized by IR and 1H, 11B, and 13C NMR spectroscopy in solution, and the solid-state structures were established by X-ray crystallographic analysis.