Despite the abundance of f-block-cyclopentadienyl, arene, cycloheptatrienyl and cyclooctatetraenide complexes, cyclobutadienyl derivatives are unknown in spite of their prevalence in the d-block. Here we report that reductive [2 þ 2]-cycloaddition reactions of diphenylacetylene and (2,2-dimethylpropylidyne)phosphine with uranium(V)-inverted sandwich 10p-toluene tetra-anion complexes results in the isolation of inverted sandwich cyclobutadienyl and diphosphacyclobutadienyl dianion uranium(IV)

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... plexes. Computational analysis suggests that the bonding is predominantly electrostatic. Although the c 4 molecular orbital in the cyclobutadienyl and diphosphacyclobutadienyl ligands exhibits the correct symmetry for d-bonding to uranium, the dominant covalent contributions arise from p-bonding involving c 2 and c 3 orbital combinations. This contrasts with uranium complexes of larger arenes and cyclo-octatetraenide, where d-bonding dominates. This suggests that the angular requirements for uranium to bond to a small four-membered ring favours p-bonding, utilizing 5f-instead of 6d-orbitals, over d-bonding that is favoured with larger ligands, where 6d-orbitals can become involved in the bonding.