The occurrence of the monohydrated hydroxide as a bridging ligand between copper(II) ions in a complex is reported here for the first time. The crystal structure of 1 is made up of one-dimensional bpym-bridged copper(II) chains which are linked by the H3O2- group to yield a 2D-network. Magnetic susceptibility measurements reveal the existence of a giant antiferromagnetic interaction (the compound is diamagnetic) between copper(II) ions through the bihydroxide bridge. An orbital interpretation of such a remarkable efficiency of this ligand to transmit electronic effects between paramagnetic centers is suggested.