We found no evidence of pressure-induced framework degradation or phase transitions up to 0.4 GPa. Fitting of the pressure dependence of the volume using the second-order Birch-Murnaghan equation of state was carried out using the PAS-Cal Python package , and revealed that the bulk compressibility was B 0 = 18.7(3) GPa for the pure NiCl 2 (btd), with the two doped samples both slightly stiffer: x = 0.10 has B 0 = 20.6(0.3) GPa and x = 0.22 has B 0 = 19.96( ) GPa [Fig. ]. These values are comparable to those reported for other nickel(II) layered materials, e.g. Ni(NCS) 2 B 0 = 17.0(2) GPa, and NiI 2 B 0 = 27.7(9) GPa, and stiffer than ZnCl 2 (3,5-dichloropyridine) 2 , B 0 = 14.52 (8), which contains 1D ZnCl 2 chains. Our X-ray diffraction measurements allow us to determine not only the bulk modulus, but how the compressibility varies with direction. We find that the principal compressibilities do approximately coincide with the crystallographic axes, although in a monoclinic system the principal strains will not lie in general, along the unit cell axes. The compressibility is largest along the interlayer direction, X 1 (~c) K 1 = 27.3(3) TPa -1 . X 2 (~a) is next stiffest, corresponding to the Ni-Cl-Ni chain direction, K 2 = 14.8(4) TPa -1 , with the stiffest direction being the X 3 (b) along the Ni-N-(O/S)-N-Ni bonds direction, K 3 = 7.7(4) TPa -1 [Fig. ), ESI Table ]. The softness of X 1 is typical of vdW layered materials, e.g. Ni(NCS) 2 K vdW = 32.5(2) TPa -1 , where K vdW is the compressibility normal to the vdw layers. As inorganic materials tend to be less compressible, the direction with purely inorganic connectivity might be expected to be the stiffest, but in fact it is nearly twice as soft as the direction with purely metal-organic connectivity. This perhaps is due to the fact that the X 3 direction primarily involves direct bond compression, whereas the X 2 direction corresponds to bending of the Ni-Cl-Ni angle, although it is notable that DFT calculations suggest significant ππ-interactions between the organic ligands along this direction which may modulate the compressibility. This trend is consistent with previous investigations of metal organic materials, such as in [CuCl(pyrazine) 2 ]BF 4 where the Cu-pyrazine-Cu plane was significantly stiffer than the Cu-Cl-Cu chains, and the plastically deforming ZnCl 2 (3,5-dichloropyridine) 2 , where the ZnCl 2 chains are as soft as the vdW directions (K ZnCl2 ≈ 23 TPa -1 ).