Among the polyhedral [closo-B_nH_n]²⁻ ion series (n = 5–12 inclusive) the aromatic [closo-B₁₀H₁₀]²⁻ ion is both readily available and quite reactive. Among its many reactions which retain its cage structure one finds the oxidative dimerization reaction in which two [closo-B₁₀H₁₂]²⁻ ions each formally lose a hydride ion and undergo dimerization of the resulting [closo-B₁₀H₉]⁻ ions to produce the [trans-B₂₀H₁₈]²⁻ ion. The two-component [closo-B₁₀H₉]⁻ ions of the latter are linked together by a pair of unique B–B–B bonds which provide unprecedented reactivity to the structure. Among these reactions are the two-electron reduction to a set of three interconvertible [B₂₀H₁₈]⁴⁻ ions having intercage B–B bonds and the related reductive substitution reaction in which [trans-B₂₀H₁₈]²⁻ undergoes attack by nucleophile, L, to produce [B₂₀H₁₈L]²⁻. The latter species is formally a substituted [B₂₀H₁₉]³⁻ (L = H) ion formed by B–B bond protonation of one of the isomeric [B₂₀H₁₈]⁴⁻ ions. These and a variety of novel reactions are described here along with interrelated reaction mechanisms considered for the first time.