Electron-density analysis of halide⋯halide through-space magnetic exchange

We present a combined experimental and density functional theory study that characterizes the charge and spin density in NiX₂(3,5-lutidine)₄ (X = Cl, Br and I). In this material, magnetic exchange interactions occur via Ni²⁺–halide⋯halide–Ni²⁺ pathways, forming one-dimensional chains. We find evidence for weak halide⋯halide covalency in the iodine system, which is greatly reduced when X = Br and is absent for X = Cl; this is consistent with the reported `switching-on' of magnetic exchange in the larger-halide cases. Our results are benchmarked against density functional theory calculations on [NiHF₂(pyrazine)₂]SbF₆, in which the primary magnetic exchange is mediated by F–H–F bridging ligands. This comparison indicates that, despite the largely depleted charge density found at the centre of halide⋯halide bonds, these through-space interactions can support strong magnetic exchange gated by weak covalency and enhanced by significant electron density overlapping that of the transition metal centres.