The Emergence of Actinide Cyclobutadienyl Chemistry

Since its inception in the 1950s, the field of organoactinide chemistry has developed and is still burgeoning today. A plethora of molecular actinide cyclopentadienyl (C-5), arene (C-6), cycloheptatrienyl (C-7), and cyclooctatetraenyl (C-8) complexes are known. However, the first f-element cyclobutadienyl complex, a uranium derivative, was only reported as recently as 2013, which contrasts to transition metal chemistry where the first cyclobutadienyl derivatives were realised in the 1950s. A small but growing number of uranium and thorium cyclobutadienyl complexes are now known, so now is an opportune time to review progress to date. This Minireview addresses bonding considerations for the cyclobutadienyl ligand, surveys synthetic routes to crystallographically authenticated actinide cyclobutadienyl complexes and their novel bonding features, and highlights future directions that merit development.