Exo-metal coordination by a tricyclic [{P(µ-N-2-NC5H4)}2(µ-O)]2 dimer in [{P(µ-N-2-NC5H4)}2(µ-O)]2{CuCl·(C5H5N)2}4{2-NC5H4= 2-pyridyl, C5H5N = pyridine)