catena -Poly[[μ 3 -hydroxido-tetra-μ 2 -pyridazine-1:2κ 4N : N ′;1:3κ 2N : N ′;2:3κ 2N : N ′-tetrakis(selenocyanato)-1κ N ,2κ N ,3κ 2N -trizinc(II)]-μ-cyanido-1:2′κ 2C : N ]