2,2':6',2'':6'',2'''-Quaterpyridine (qtpy): A versatile ligand in metallosupramolecular chemistry; Crystal and molecular structures of [Ni(qtpy)(OH2)(2)][BF4](2), [Pd(qtpy)][PF6](2), [Cu-2(qtpy)(2)][PF6](2) and [Ag-2(qtpy)(2)][BF4](2)

The co-ordination behaviour of 2,2':6',2 '':6 '',2'''-quaterpyridine (qtpy) has been systematically studied. Double-helical dinuclear complexes were only obtained with ions bearing a low charge or a low charge-to-radius ratio and which have no electronically imposed preference for geometries in which the ligand can present a planar donor set. The double-helical complexes [Cu-2(qtpy)(2)][PF6](2) and [Ag-2(qtpy)(2)][BF4](2) have been structurally characterised, as have the mononuclear species [Ni(qtpy)(OH2)(2)][BF4](2) and [Pd(qtpy)][PF6](2). Complexes with zinc(II) and cadmium(II) are mononuclear but it is suggested that a double-helical dinuclear complex is formed with mercury(II).