K. Wade summarized the relation of the number of valence electrons used for
skeletal bonds and the structures of boranes and proposed an empirical rule called the
Wade rule. According to this rule, when the number of boron atoms is n, the skeletal
valence electrons are 2(n+1) for a closo type, 2(n+2) for a nido type, and 2(n+3) for an
arachno type borane. The relationship between the skeletal structure of a cluster
compound and the number of valence electrons is also an important problem in the cluster
compounds of transition metals, and the Wade rule has played a significant role in
furthering our understanding of the structures of these compounds.