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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 whenX= Br and is absent forX= 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. 

The quasi-one-dimensional molecule-based Heisenberg antiferromagnet Cu(NO 3 ) 2 (pyz) 3 has an intrachain coupling J = 13.7(1) K ( ) and exhibits a state of long-range magnetic order below T N = 0.105(1) K. The ratio of interchain to intrachain coupling is estimated to be | J ′/ J | = 3.3 × 10 −3 , demonstrating a high degree of isolation for the Cu chains.