“…In general, given the peculiar nature of the apical atom, one can discriminate four different types of such complexes: (1) cyclobutadiene complexes of the main group elements: s-block elements (alkali and alkaline earth metals) and p-block elements, (2) cyclobutadiene complexes of d-block elements (transition metals); (3) cyclobutadiene complexes of f-block elements (lanthanides and actinides). However, if numerous cyclobutadiene complexes of the elements of s-block (including tetrasila- and tetragermacyclobutadiene analogues), 33–38 d-block (including tetrasila- and tetragermacyclobutadiene analogues), 37,39–47 and f-block 48–59 were reported, cyclobutadiene complexes of the p-block elements, which are commonly known as “pyramidanes”, were without precedents until recently. 60…”