Reactions of a Zn-Zn bond with main group carbene analogues as a prototypical case of reductive addition.

Oxidative addition most commonly involves the addition of a substrate to a metal centre. This reaction is fundamental across synthetic chemistry and underpins numerous catalytic methods. In the textbook description of oxidative addition reactions, a net increase in the formal oxidation state of the metal occurs with simultaneous bond breaking at the substrate. The majority of known oxidative addition reactions, however, involve substrates bearing relatively electronegative elements (for example, hydrogen, carbon, nitrogen, oxygen and halogens) and there has been little discussion of how addition processes may fundamentally change if substrates were constructed from more electropositive elements. Here we show that the zinc-zinc bonded complex, CpZnZnCp (Cp* = pentamethylcyclopentadienyl), which is isoelectronic with dihydrogen, undergoes facile addition to the metal (or semi-metal) centres of a series of main group carbene analogues based on silicon, aluminium, gallium or indium. Reactions proceed with complete breaking of the zinc-zinc bond and an increase in the coordination number of the central metal from two to four. Our analysis suggests that these addition processes are not oxidative, but rather there is likely a continuum of redox outcomes spanning oxidative, redox neutral and reductive. The addition of CpZnZnCp to silicon(II) provides the most compelling case for a prototypical reductive addition process.