A few principles of how composition determines the mechanical properties of molecular perovskites have already been established. For instance, nanoindentation and highpressure diffraction studies on materials such as [(CH3)2NH2]M(HCOO)3 (with M 2+ = Ni 2+ , Co 2+ , Zn 2+ and Mn 2+ ), [C(NH2)3]M(HCOO)3 (with M 2+ = Co 2+ and Mn 2+ ) and [Pr4N]M(C2N3)3 (with [Pr4N] + = tetrapropylammonium, M 2+ = Ni 2+ , Co 2+ and Mn 2+ ) have demonstrated that a smaller size of M 2+ paired with a larger ligand field stabilisation energy (LFSE) leads to a larger Young's modulus and B. These findings agree with our recent results, where we observed a decreasing B as a function of increasing Shannon ionic radius of M 2+ . Looking at the role of A + , its capability to form hydrogen bonding interactions (HBIs) to the [MX3] -network has been found to be an important factor. For instance, the mechanical properties are tunable across the solid solution [NH3NH2]1-x[NH3OH]xZn(HCOO)3 via HBIs, i.e. the choice of A + . More recently, this strategy has been used to introduce ferroelasticity in [Et3P(CH2)2Cl]1-x[Et3P(CH2)2F]xMn(C2N3)3. Yet, our understanding of the interplay between the A + and active framework distortions of the [MX3] -is only developing. In the pursuit to fill this gap, we have synthesised a new series of ). The absence of acidic hydrogen atoms in all A + excludes contributions from HBIs. Furthermore, by keeping M 2+ = Ni 2+ in the series, we have minimised the effect of M 2+ cation size and LFSE, and hence, the chemical bond strength in the [Ni(C2N3)3] - network on the structure and properties of the molecular perovskites. Therefore, this series allows us to isolate the impact of the size and shape of the A + cation on the distortions of the 3D [MX3] -network and their ramifications on the mechanical properties, which we measured via high-pressure powder X-ray diffraction (HPPXRD) by using B as a proxy. In addition -by employing a solid solution approach -we investigated to which extent B can be fine-tuned via A + substitution.