We note an opposite evolution of the adiabatic energy difference for the four gas molecules between the Ni(II) case and Fe(II) and Mn(II). For Fe and Mn, larger values of adiabatic energy differences are found for molecules exhibiting stronger-field ligands, i.e. for a higher destabilization of d z 2 with respect to the low lying non-bonding orbitals. Thus, a LS→HS change involving a change in occupation between the d z 2 and dxy, results in the largest values of ∆EH-L for CO in Mn (and Fe eventually) and the smallest when Ni is considered (see illustration of the molecular-orbital dia-gram in Fig. ). For the same reason, increasing values of ligand fields result in the progressive decrease of ∆EH-L in the case of Ni(II). In this case, the destabilization of d z 2 upon (strong) binding brings this molecular orbital closer to high-lying antibonding d x 2 -y 2 , and because the spin transition involves these two energy levels, the associated ∆EH-L decreases.