Bis-bidentate vs. bis-tridentate imino-heterocycle ligands in the formation of dinuclear helical complexes of Fe(ii)Electronic supplementary information (ESI) available: atomic positions, bond lengths and angles, anisotropic thermal parameters, hydrogen atom coordinates, data collection, and crystal parameters for all crystallographically characterized complexes; NMR spectra, including COSY, and CD spectra for ligand 3 and [FeII2(3)2]4+. See http://www.rsc.org/suppdata/dt/b2/b210137h/