Cadmium(II) chloride, bromide and iodide complexes with 4,4′-bipyrid-azine: When are diazine and halide bridges (in)compatible?

In poly[di - chlorido - (4,4′-bipyridazine)-² N ¹:N ^1′-cad-mium(II)], [CdCl2(C8H6N4)] n , (I), and its isomorphous bromide analogue, [CdBr2(C8H6N4)] n , (II), the halide atom lies on a mirror plane and the Cd^II ion resides at the inter-section of two perpendicular mirror planes with m2m site symmetry. The pyridazine rings of the ligand lie in a mirror plane and are related to each other by a second mirror plane perpendicular to the first. The compounds adopt the characteristic structure of the [M ^II X 2(bipy)] type (bipy is bipyridine) based on crosslinking of [Cd(-X)2] n chains [Cd - Cl = 2.5955 (9) and 2.6688 (9) Å; Cd - Br = 2.7089 (4) and 2.8041 (3) Å] by bitopic rod-like organic ligands [Cd - N = 2.368 (3)-2.380 (3) Å]. This feature is discussed in terms of supra-molecular stabilization, implying that the periodicity of the inorganic chain [Cd⋯Cd = 3.7802 (4) Å in (I) and 3.9432 (3) Å in (II)] is favourable for extensive parallel π-π stacking of monodentate pyridazine rings, with centroid-centroid distances of 3.7751 (4) Å in (I) and 3.9359 (4) Å in (II). This is not the case for the longer iodide bridges, which cannot stabilize such a pattern. In poly[tetra - iodido-4-(4,4′-bipyridazine)-⁴ N ¹:N ²:N ^1′:N ^2′-di-cadmium(II)], [Cd2I4(C8H6N4)] n , (III), the ligands are situated across a centre of inversion; they are tetra-dentate [Cd - N = 2.488 (2) and 2.516 (2) Å] and link successive [Cd(-I)2] n chains [Cd - I = 2.8816 (3)-3.0069 (4) Å] into corrugated layers.