Cyclobutadiene (and Its Higher Homologues) Complexes of the s-, p-, d-, and f-Block Elements

Abstract Square-planar triplet cyclobutadiene is intrinsically unstable, given its four π-electron configuration leading to unavoidable antiaromatic destabilization. One effective way to stabilize cyclobutadiene is through its incorporation into the coordination sphere of an element capable of electron donation. Metals (main group metals, transition metals, lanthanides, and actinides) are most frequently used as electron-donating elements, although recently, it was established that the main group elements may also serve as capping donors in cyclobutadiene complexes. The recent advances in the chemistry of the cyclobutadiene complexes of the s-, p-, d-, and f-block elements are briefly summarized in this review.