Ligand cooperation has become an important strategy in the development of new transition metal based transformations. By using this approach some remarkable new catalytic transformations have been achieved, all in the space of only a few years. The purpose of this tutorial review is to explore the potential utilisation of ligands containing borohydride and borane functionalities as reversible hydrogen atom stores. At the heart of this review will be a discussion on hydrogen transfer reactions and the transformation between borohydride and borane moieties. An outline of the various synthetic routes to metal–borane (metallaboratrane) complexes will be provided together with a discussion of their further reactivity including key transformations such as 1,2 additions across the metal–boron bond and ‘recharging’ the borane functional group back to borohydride. Finally, an evaluation of the potential future applications of such reactivity will be provided.

The addition of H₂ across a transition metal–borane bond is reported for the first time providing a mechanism for recharging borane functional groups to borohydride.

This feature article examines some of the recent advances in the chemistry of Z-type transition metal–borane and X-type transition metal–boryl complexes. It focuses on the employment of these boron-based functionalities acting as stores and transfer agents for functional groups such as hydrides, alkyl groups and aryl groups which can either be abstracted or delivered to the metal centre. The review also explores the rather novel reactivity involving the cleavage of H–H, E–H and E–E′ bonds (where E and E′ are a range of groups) across the transition metal–boron bond in such complexes. It explores the early examples of the addition of H–H across transition metal–borane bonds and describes the new transformation in the context of other known modes of hydrogen activation including classic oxidative addition and heterolytic cleavage at transition metal centres as well as Frustrated Lewis Pair chemistry. Similar reactivity involving transition metal–boryl complexes are also described particularly those which undergo both boryl-to-borane and borane-to-borohydride transformations. The delivery of hydride to the metal centre in combination with the potential to regenerate the borohydride functional group via a recharging process is explored in the context of providing a new strategy for catalysis. Finally, a light-hearted look at the analogy of the ‘stinging processes’ involving Trofimenko type ligands is taken one step further to determine whether it is indeed in the nature of scorpionate ligands to repeatedly ‘sting’ just as the real life scorpions do.