The isolation and structural characterization of 1 : 1 adducts of copper(1) chloride (1) and bromide (2) with pyridine-4-carbonitrile (L) is described; crystals of the two complexes are isomorphous (monoclinic, P21/c, a 蠳.9, b 蠱4.7, c 蠱3.0 Š, ߠ蠹6ଠZ 4; R0.047, 0.063 for No 630, 707 'observed' reflections respectively). Unlike the 1 : 1 adducts with the parent pyridine and benzonitrile ligands which are 'stair' polymers, these complexes comprise 'split-stair' strands woven into a two-dimensional sheet by crosslinking ambidentate ligands. Cu-N ( nitrile ) (1.942(9), 1.96(1) Š) are appreciably shorter than Cu-N (pyridine) (2.066(8), 2.04(1) Š), as in the parent base complexes. The two Cu-X are similar in each case: 2.336(6), 2.366(3) Š(CI); 2.460(3), 2.486(3) Š(Br). The iodide adduct (3) isolated is of novel stoichiometry (Cul : L, 4 : 5) (monoclinic, P21/c; a 10.140(5), b 12.214(6), c 15.157(8) Š, ߠ99.99(4)ࠚ 2; R 0.048, No 2416). It is a linear polymer, comprising tetranuclear Cu414L4 'step' units, crosslinked across inversion centres by disordered ambidentate ligands.