We employ an ab initio electronic structure computational scheme to provide design principles towards the development of spin-crossover assisted gas release. For this, we perform an extensive computational study of the relationship between the spin state of the open metal of seven different porous materials and the affinity for the host molecules. Two different mechanisms are studied. In the case of elongated octahedral complexes, the first occurs upon a transition from low spin to intermediate spin. This mechanism, already discussed in previous studies, involves a change in the bond order between the metal and the molecule as a consequence of the change in the occupation of the d z 2 orbital. The second occurs upon a transition from intermediate spin to high spin, it is of electrostatic nature, and arises from the change in the occupation of between the antibonding d x 2 -y 2 and one non-bonding d xz/yz orbital. The latter, which was not reported before but finds confirmation in previous experimental and computational studies, is substantially less effective in modifying the binding energy than the former.