Homogeneous catalysis with Earth-abundant, first-row transition metals, including iron and cobalt, has gained considerable recent attention as a potentially cost-effective and sustainable alternative to more commonly and historically used precious metals. Because fundamental organometallic transformations, such as oxidative addition and reductive elimination, are two-electron processes and essential steps in many important catalytic cycles, controlling redox chemistry, in particular overcoming one-electron chemistry, has been as a central challenge with Earth-abundant metals. This Perspective focuses on approaches to impart sufficiently strong-ligand fields to generate electron-rich metal complexes able to promote oxidative addition reactions where the redox changes are exclusively metal-based. Emphasis is placed on how ligand design and exploration of fundamental organometallic chemistry coupled with mechanistic understanding has been used to discover iron catalysts for the hydrogen isotope exchange in pharmaceuticals and cobalt catalysts for C(sp 2)─H borylation reactions. A pervasive theme is that first-row metal complexes often promote unique chemistry from their precious-metal counterparts, demonstrating that these elements offer a host of new opportunities for reaction discovery and for sustainable catalysis.

The first examples for the rhenium-catalysed hydroboration of aldehydes, ketones and aldimines, including heteroaromatic quinoline, are reported herein. Reactions are remarkably chemoselective and tolerant of several functional groups. A wide array of rhenium complexes were efficient pre-catalysts for these hydroborations, including new low-valent complexes of the formula [Re(N-N)(CO)(L)]X (N-N = bipy derivative, L = labile ligand/solvent, and X = [BAr] and [B(3,5-di-tBu-cat)]), which have been characterized fully including an X-ray diffraction study for [Re(bipy)(CO)(quin)][BAr] (2). A new silver spiroboronate ester Ag[B(3,5-di-tBu-cat)](NCCH) (3) was prepared and characterized fully, including an X-ray diffraction study, and used to make one of the new rhenium complexes.

Deprotonation of the dimethyl sulfide ligand in [Re(bipy)(CO)3(SMe2)][OTf] (1) by KN(SiMe3)2 afforded a mixture of two diastereomers (2M and 2m) in which a C-C bond has been formed between the S-bonded CH2 group and the 2 position of 2,2'-bipyridine. The solid-state structure of the more stable 2,6-(i)Pr-BIAN analogue could be determined by X-ray diffraction.

Well-defined bis(silylene)pyridine cobalt(III) precatalysts for C(sp 2 )−H borylation have been synthesized and applied to the investigation of the mechanism of the catalytic borylation of furans and 2,6-lutidine. Specifically, [(

KN(SiMe3)2 reacts with [Re(CO)3(phen)(PMe3)]OTf via reversible addition to the phen ligand and irreversible deprotonation of the PMe3 ligand followed by intramolecular attack to phen by the deprotonated phosphane, while MeLi irreversibly adds to phen. The addition of MeLi has been shown to be intermolecular, unlike previously known nucleophilic additions to pyridines.