The crystal structures of Ni$X_2$(pyz)$_2$ ($X$ = Cl (textbf{1}), Br (textbf{2}), I (textbf{3}) and NCS (textbf{4})) were determined at 298~K by synchrotron X-ray powder diffraction. All four compounds consist of two-dimensional (2D) square arrays se
lf-assembled from octahedral NiN$_4$$X_2$ units that are bridged by pyz ligands. The 2D layered motifs displayed by textbf{1}-textbf{4} are relevant to bifluoride-bridged [Ni(HF$_2$)(pyz)$_2$]$Z$F$_6$ ($Z$ = P, Sb) which also possess the same 2D layers. In contrast, terminal $X$ ligands occupy axial positions in textbf{1}-textbf{4} and cause a staggering of adjacent layers. Long-range antiferromagnetic order occurs below 1.5 (Cl), 1.9 (Br and NCS) and 2.5~K (I) as determined by heat capacity and muon-spin relaxation. The single-ion anisotropy and $g$ factor of textbf{2}, textbf{3} and textbf{4} are measured by electron spin resonance where no zero--field splitting was found. The magnetism of textbf{1}-textbf{4} crosses a spectrum from quasi-two-dimensional to three-dimensional antiferromagnetism. An excellent agreement was found between the pulsed-field magnetization, magnetic susceptibility and $T_textrm{N}$ of textbf{2} and textbf{4}. Magnetization curves for textbf{2} and textbf{4} calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. textbf{3} is characterized as a three-dimensional antiferromagnet with the interlayer interaction ($J_perp$) slightly stronger than the interaction within the two-dimensional [Ni(pyz)$_2$]$^{2+}$ square planes ($J_textrm{pyz}$).
Although the isotope effect in superconducting materials is well-documented, changes in the magnetic properties of antiferromagnets due to isotopic substitution are seldom discussed and remain poorly understood. This is perhaps surprising given the p
ossible link between the quasi-two-dimensional (Q2D) antiferromagnetic and superconducting phases of the layered cuprates. Here we report the experimental observation of shifts in the N{e}el temperature and critical magnetic fields ($Delta T_{rm N}/T_{rm N}approx 4%$; $Delta B_{rm c}/B_{rm c}approx 4%$) in a Q2D organic molecular antiferromagnets on substitution of hydrogen for deuterium. These compounds are characterized by strong hydrogen bonds through which the dominant superexchange is mediated. We evaluate how the in-plane and inter-plane exchange energies evolve as the hydrogens on different ligands are substituted, and suggest a possible mechanism for this effect in terms of the relative exchange efficiency of hydrogen and deuterium bonds.
