Dihydrogenated boron clusters, H2Bn- (n = 7-12), were produced and characterized using photoelectron spectroscopy and computational chemistry to have ladderlike structures terminated by a hydrogen atom on each end. The two rows of boron atoms in the dihydrides are bonded by delocalized three-, four-, or five-center sigma and pi bonds. The pi bonding patterns in these boron nanoladders bear similarities to those in conjugated alkenes: H2B7-, H2B8, and H2B9-, each with two pi bonds, are similar to butadiene, while H2B102-, H2B11-, and H2B12, each with three pi bonds, are analogous to 1,3,5-hexatriene. The boron cluster dihydrides can thus be considered as polyene analogues, or "polyboroenes". Long polyboroenes with conjugated pi bonds (analogous to polyacetylenes), which may form a new class of molecular wires, should exist.