Synthesis and reactivity of low-oxidation-state alkaline earth metal complexes

The synthesis of molecules that feature main-group elements in unusual oxidation states and coordination environments is a primary pursuit of main-group chemistry. The p-block elements saw early success towards this goal, and dozens of compounds that contain subvalent p-block metals, semi-metals and non-metals are now known. The development of reliable syntheses for these compounds made it possible to study them in detail, which expanded our understanding of bonding and electronic structure and served as the foundation from which catalysis mediated by main-group elements has emerged. For the group 2 elements, isolating reduced compounds has been a synthetic challenge that has spurred exciting progress in the synthesis of reduced alkaline earth compounds. The past two decades has seen the isolation of stable Be(0), Be(I), Mg(0), Mg(I) and Ca(I) compounds, along with studies of their reactivity profiles. In this Review, we overview the chemistry of isolated low-valent species with a focus on comparing newly discovered chemical trends and features among the different elements in the group. Finally, we discuss future directions and challenges for the field.