The stabilities of about 25 electronic states of B21 have been investigated using a multireference CI (MRD-CI) method and an AO basis set composed of 6s4p2d contracted Gaussian species per atom, including semidiffuse functions relevant for an adequate description of charge transfer interactions. The ground state X 1Σg + (σ2σu 2) is repulsive, as expected by its electronic configuration with a zero bond order. In spite of this and the doubly-positive charge, many excited states are found to be metastable, four of them (11Πg, 11Πu, 1 3Σg -, and 11Δ g) having potential wells from 1.52 eV (11Π u) to 2.83 eV (13Σg -). Relative to the ground state configuration, the metastable states arise from the excitations σu → σg(3Σ u +), σu→u( 3,1Πg), σu 2, → σgπu(3,1Πu), and σu 2 → πu 2( 3Σ-, 1Δg, 1Σg +); they are analogous to those states showing deep local minima in B2 and B2 +. Differences in stabilities among quasibound states can be explained on the basis of the asymptotic ΔE's between repulsive channels B+ + B + and appropriate higher-lying limits (states) B+B2+ of bonding character; another important factor governing stability is the actual bonding character of the electronic configurations assigned to each state. The vertical double-ionization potential B2 → B2 2+(πu 2→∞) between both ground states is 27.97 eV, the repulsive X 1Σg + dication state being created with 8.37 eV excess energy relative to B + + B+. Doubly-ionized states with a chance of being detected because of their long lifetimes against predissociation are those showing a strongly quasibound character, such as (with the vertical double ionization potential in eV given in parentheses): 11Π g(σuπu → ∞; 30.29); 1 1Πu(σu2πu → σg∞; 31.30); 13Σg -(σu 2 → ∞; 31.33), and 1 1Δg(σu 2 → ∞; 31.95).