The Because the d z 2 has an antibonding character along z, the spin crossover-induced change in the occupation of this molecular orbital yields a change in the bond order of the gas. This decrease in binding energy and consequent bond elongation have been discussed previously in the case of gas adsorption in BTT-MOFs and MOF-74 where the change in occupation of the d z 2 resulted from the increase in the electron count of the d metal in MOFs is HS rather than from a spin crossover. This case applies to the change between S=1 and S=0 for Fe(II); between S=3/2 and S=1/2 for Fe(III); and between S=1 and S=0 for Ni(II). See Table upon LS→HS transition. For clarity, the change in binding energy is reported with an opposite sign in the case of Ni(II) in Figure : in this case, the LS→HS transition is accompanied by an increase in binding energy which is consistent with a decrease in the occupation number of the d z 2 for a metal with a d 8 electron count. We note a smoother evolution of the curve for Ni(II) and a systematically larger change for Fe(II) with respect to Fe(III): while the binding energy is similar for the low spin case, the intermediate spin case exhibits systematically larger binding in the case of Fe(III). For CO for example, the binding energy for LS is 2.159 eV for Fe(II) and 2.270 eV for Fe(III), and for IS we predict 0.660 eV and 1.330, respectively. This results in a change of 1.50 eV for Fe(II) and 0.94 eV for Fe(III) upon spin change. We attribute this to the larger electrostatic charges computed on the Fe(III) metal which may affect the binding more when weak interactions dominate (in IS).